Network Working Group R. Housley
Request for Comments: 2459 SPYRUS
Category: Standards Track W. Ford
VeriSign
W. Polk
NIST
D. Solo
Citicorp
January 1999
Internet X.509 Public Key InfrastructureCertificate and CRL Profile
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (1999). All Rights Reserved.
Abstract
This memo profiles the X.509 v3 certificate and X.509 v2 CRL for use
in the Internet. An overview of the approach and model are provided
as an introduction. The X.509 v3 certificate format is described in
detail, with additional information regarding the format and
semantics of Internet name forms (e.g., IP addresses). Standard
certificate extensions are described and one new Internet-specific
extension is defined. A required set of certificate extensions is
specified. The X.509 v2 CRL format is described and a required
extension set is defined as well. An algorithm for X.509 certificate
path validation is described. Supplemental information is provided
describing the format of public keys and digital signatures in X.509
certificates for common Internet public key encryption algorithms
(i.e., RSA, DSA, and Diffie-Hellman). ASN.1 modules and examples are
provided in the appendices.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119.
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RFC 2459 Internet X.509 Public Key Infrastructure January 19991 Introduction
This specification is one part of a family of standards for the X.509
Public Key Infrastructure (PKI) for the Internet. This specification
is a standalone document; implementations of this standard may
proceed independent from the other parts.
This specification profiles the format and semantics of certificates
and certificate revocation lists for the Internet PKI. Procedures
are described for processing of certification paths in the Internet
environment. Encoding rules are provided for popular cryptographic
algorithms. Finally, ASN.1 modules are provided in the appendices
for all data structures defined or referenced.
The specification describes the requirements which inspire the
creation of this document and the assumptions which affect its scope
in Section 2. Section 3 presents an architectural model and
describes its relationship to previous IETF and ISO/IEC/ITU
standards. In particular, this document's relationship with the IETF
PEM specifications and the ISO/IEC/ITU X.509 documents are described.
The specification profiles the X.509 version 3 certificate in Section4, and the X.509 version 2 certificate revocation list (CRL) in
Section 5. The profiles include the identification of ISO/IEC/ITU and
ANSI extensions which may be useful in the Internet PKI. The profiles
are presented in the 1988 Abstract Syntax Notation One (ASN.1) rather
than the 1994 syntax used in the ISO/IEC/ITU standards.
This specification also includes path validation procedures in
Section 6. These procedures are based upon the ISO/IEC/ITU
definition, but the presentation assumes one or more self-signed
trusted CA certificates. Implementations are required to derive the
same results but are not required to use the specified procedures.
Section 7 of the specification describes procedures for
identification and encoding of public key materials and digital
signatures. Implementations are not required to use any particular
cryptographic algorithms. However, conforming implementations which
use the identified algorithms are required to identify and encode the
public key materials and digital signatures as described.
Finally, four appendices are provided to aid implementers. AppendixA contains all ASN.1 structures defined or referenced within this
specification. As above, the material is presented in the 1988
Abstract Syntax Notation One (ASN.1) rather than the 1994 syntax.
Appendix B contains the same information in the 1994 ASN.1 notation
as a service to implementers using updated toolsets. However,
Appendix A takes precedence in case of conflict. Appendix C contains
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RFC 2459 Internet X.509 Public Key Infrastructure January 1999
notes on less familiar features of the ASN.1 notation used within
this specification. Appendix D contains examples of a conforming
certificate and a conforming CRL.
2 Requirements and Assumptions
The goal of this specification is to develop a profile to facilitate
the use of X.509 certificates within Internet applications for those
communities wishing to make use of X.509 technology. Such
applications may include WWW, electronic mail, user authentication,
and IPsec. In order to relieve some of the obstacles to using X.509
certificates, this document defines a profile to promote the
development of certificate management systems; development of
application tools; and interoperability determined by policy.
Some communities will need to supplement, or possibly replace, this
profile in order to meet the requirements of specialized application
domains or environments with additional authorization, assurance, or
operational requirements. However, for basic applications, common
representations of frequently used attributes are defined so that
application developers can obtain necessary information without
regard to the issuer of a particular certificate or certificate
revocation list (CRL).
A certificate user should review the certificate policy generated by
the certification authority (CA) before relying on the authentication
or non-repudiation services associated with the public key in a
particular certificate. To this end, this standard does not
prescribe legally binding rules or duties.
As supplemental authorization and attribute management tools emerge,
such as attribute certificates, it may be appropriate to limit the
authenticated attributes that are included in a certificate. These
other management tools may provide more appropriate methods of
conveying many authenticated attributes.
2.1 Communication and Topology
The users of certificates will operate in a wide range of
environments with respect to their communication topology, especially
users of secure electronic mail. This profile supports users without
high bandwidth, real-time IP connectivity, or high connection
availability. In addition, the profile allows for the presence of
firewall or other filtered communication.
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RFC 2459 Internet X.509 Public Key Infrastructure January 1999
This profile does not assume the deployment of an X.500 Directory
system. The profile does not prohibit the use of an X.500 Directory,
but other means of distributing certificates and certificate
revocation lists (CRLs) may be used.
2.2 Acceptability Criteria
The goal of the Internet Public Key Infrastructure (PKI) is to meet
the needs of deterministic, automated identification, authentication,
access control, and authorization functions. Support for these
services determines the attributes contained in the certificate as
well as the ancillary control information in the certificate such as
policy data and certification path constraints.
2.3 User Expectations
Users of the Internet PKI are people and processes who use client
software and are the subjects named in certificates. These uses
include readers and writers of electronic mail, the clients for WWW
browsers, WWW servers, and the key manager for IPsec within a router.
This profile recognizes the limitations of the platforms these users
employ and the limitations in sophistication and attentiveness of the
users themselves. This manifests itself in minimal user
configuration responsibility (e.g., trusted CA keys, rules), explicit
platform usage constraints within the certificate, certification path
constraints which shield the user from many malicious actions, and
applications which sensibly automate validation functions.
2.4 Administrator Expectations
As with user expectations, the Internet PKI profile is structured to
support the individuals who generally operate CAs. Providing
administrators with unbounded choices increases the chances that a
subtle CA administrator mistake will result in broad compromise.
Also, unbounded choices greatly complicate the software that shall
process and validate the certificates created by the CA.
3 Overview of Approach
Following is a simplified view of the architectural model assumed by
the PKIX specifications.
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RFC 2459 Internet X.509 Public Key Infrastructure January 19993.1 X.509 Version 3 Certificate
Users of a public key shall be confident that the associated private
key is owned by the correct remote subject (person or system) with
which an encryption or digital signature mechanism will be used.
This confidence is obtained through the use of public key
certificates, which are data structures that bind public key values
to subjects. The binding is asserted by having a trusted CA
digitally sign each certificate. The CA may base this assertion upon
technical means (a.k.a., proof of posession through a challenge-
response protocol), presentation of the private key, or on an
assertion by the subject. A certificate has a limited valid lifetime
which is indicated in its signed contents. Because a certificate's
signature and timeliness can be independently checked by a
certificate-using client, certificates can be distributed via
untrusted communications and server systems, and can be cached in
unsecured storage in certificate-using systems.
ITU-T X.509 (formerly CCITT X.509) or ISO/IEC/ITU 9594-8, which was
first published in 1988 as part of the X.500 Directory
recommendations, defines a standard certificate format [X.509]. The
certificate format in the 1988 standard is called the version 1 (v1)
format. When X.500 was revised in 1993, two more fields were added,
resulting in the version 2 (v2) format. These two fields may be used
to support directory access control.
The Internet Privacy Enhanced Mail (PEM) RFCs, published in 1993,
include specifications for a public key infrastructure based on X.509
v1 certificates [RFC 1422]. The experience gained in attempts to
deploy RFC 1422 made it clear that the v1 and v2 certificate formats
are deficient in several respects. Most importantly, more fields
were needed to carry information which PEM design and implementation
experience has proven necessary. In response to these new
requirements, ISO/IEC/ITU and ANSI X9 developed the X.509 version 3
(v3) certificate format. The v3 format extends the v2 format by
adding provision for additional extension fields. Particular
extension field types may be specified in standards or may be defined
and registered by any organization or community. In June 1996,
standardization of the basic v3 format was completed [X.509].
ISO/IEC/ITU and ANSI X9 have also developed standard extensions for
use in the v3 extensions field [X.509][X9.55]. These extensions can
convey such data as additional subject identification information,
key attribute information, policy information, and certification path
constraints.
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RFC 2459 Internet X.509 Public Key Infrastructure January 1999
However, the ISO/IEC/ITU and ANSI X9 standard extensions are very
broad in their applicability. In order to develop interoperable
implementations of X.509 v3 systems for Internet use, it is necessary
to specify a profile for use of the X.509 v3 extensions tailored for
the Internet. It is one goal of this document to specify a profile
for Internet WWW, electronic mail, and IPsec applications.
Environments with additional requirements may build on this profile
or may replace it.
3.2 Certification Paths and Trust
A user of a security service requiring knowledge of a public key
generally needs to obtain and validate a certificate containing the
required public key. If the public-key user does not already hold an
assured copy of the public key of the CA that signed the certificate,
the CA's name, and related information (such as the validity period
or name constraints), then it might need an additional certificate to
obtain that public key. In general, a chain of multiple certificates
may be needed, comprising a certificate of the public key owner (the
end entity) signed by one CA, and zero or more additional
certificates of CAs signed by other CAs. Such chains, called
certification paths, are required because a public key user is only
initialized with a limited number of assured CA public keys.
There are different ways in which CAs might be configured in order
for public key users to be able to find certification paths. For
PEM, RFC 1422 defined a rigid hierarchical structure of CAs. There
are three types of PEM certification authority:
(a) Internet Policy Registration Authority (IPRA): This
authority, operated under the auspices of the Internet Society,
acts as the root of the PEM certification hierarchy at level 1.
It issues certificates only for the next level of authorities,
PCAs. All certification paths start with the IPRA.
(b) Policy Certification Authorities (PCAs): PCAs are at level 2
of the hierarchy, each PCA being certified by the IPRA. A PCA
shall establish and publish a statement of its policy with respect
to certifying users or subordinate certification authorities.
Distinct PCAs aim to satisfy different user needs. For example,
one PCA (an organizational PCA) might support the general
electronic mail needs of commercial organizations, and another PCA
(a high-assurance PCA) might have a more stringent policy designed
for satisfying legally binding digital signature requirements.
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RFC 2459 Internet X.509 Public Key Infrastructure January 1999
(c) Certification Authorities (CAs): CAs are at level 3 of the
hierarchy and can also be at lower levels. Those at level 3 are
certified by PCAs. CAs represent, for example, particular
organizations, particular organizational units (e.g., departments,
groups, sections), or particular geographical areas.
RFC 1422 furthermore has a name subordination rule which requires
that a CA can only issue certificates for entities whose names are
subordinate (in the X.500 naming tree) to the name of the CA itself.
The trust associated with a PEM certification path is implied by the
PCA name. The name subordination rule ensures that CAs below the PCA
are sensibly constrained as to the set of subordinate entities they
can certify (e.g., a CA for an organization can only certify entities
in that organization's name tree). Certificate user systems are able
to mechanically check that the name subordination rule has been
followed.
The RFC 1422 uses the X.509 v1 certificate formats. The limitations
of X.509 v1 required imposition of several structural restrictions to
clearly associate policy information or restrict the utility of
certificates. These restrictions included:
(a) a pure top-down hierarchy, with all certification paths
starting from IPRA;
(b) a naming subordination rule restricting the names of a CA's
subjects; and
(c) use of the PCA concept, which requires knowledge of individual
PCAs to be built into certificate chain verification logic.
Knowledge of individual PCAs was required to determine if a chain
could be accepted.
With X.509 v3, most of the requirements addressed by RFC 1422 can be
addressed using certificate extensions, without a need to restrict
the CA structures used. In particular, the certificate extensions
relating to certificate policies obviate the need for PCAs and the
constraint extensions obviate the need for the name subordination
rule. As a result, this document supports a more flexible
architecture, including:
(a) Certification paths may start with a public key of a CA in a
user's own domain, or with the public key of the top of a
hierarchy. Starting with the public key of a CA in a user's own
domain has certain advantages. In some environments, the local
domain is the most trusted.
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RFC 2459 Internet X.509 Public Key Infrastructure January 1999
(b) Name constraints may be imposed through explicit inclusion of
a name constraints extension in a certificate, but are not
required.
(c) Policy extensions and policy mappings replace the PCA
concept, which permits a greater degree of automation. The
application can determine if the certification path is acceptable
based on the contents of the certificates instead of a priori
knowledge of PCAs. This permits automation of certificate chain
processing.
3.3 Revocation
When a certificate is issued, it is expected to be in use for its
entire validity period. However, various circumstances may cause a
certificate to become invalid prior to the expiration of the validity
period. Such circumstances include change of name, change of
association between subject and CA (e.g., an employee terminates
employment with an organization), and compromise or suspected
compromise of the corresponding private key. Under such
circumstances, the CA needs to revoke the certificate.
X.509 defines one method of certificate revocation. This method
involves each CA periodically issuing a signed data structure called
a certificate revocation list (CRL). A CRL is a time stamped list
identifying revoked certificates which is signed by a CA and made
freely available in a public repository. Each revoked certificate is
identified in a CRL by its certificate serial number. When a
certificate-using system uses a certificate (e.g., for verifying a
remote user's digital signature), that system not only checks the
certificate signature and validity but also acquires a suitably-
recent CRL and checks that the certificate serial number is not on
that CRL. The meaning of "suitably-recent" may vary with local
policy, but it usually means the most recently-issued CRL. A CA
issues a new CRL on a regular periodic basis (e.g., hourly, daily, or
weekly). An entry is added to the CRL as part of the next update
following notification of revocation. An entry may be removed from
the CRL after appearing on one regularly scheduled CRL issued beyond
the revoked certificate's validity period.
An advantage of this revocation method is that CRLs may be
distributed by exactly the same means as certificates themselves,
namely, via untrusted communications and server systems.
One limitation of the CRL revocation method, using untrusted
communications and servers, is that the time granularity of
revocation is limited to the CRL issue period. For example, if a
revocation is reported now, that revocation will not be reliably
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RFC 2459 Internet X.509 Public Key Infrastructure January 1999
notified to certificate-using systems until the next periodic CRL is
issued -- this may be up to one hour, one day, or one week depending
on the frequency that the CA issues CRLs.
As with the X.509 v3 certificate format, in order to facilitate
interoperable implementations from multiple vendors, the X.509 v2 CRL
format needs to be profiled for Internet use. It is one goal of this
document to specify that profile. However, this profile does not
require CAs to issue CRLs. Message formats and protocols supporting
on-line revocation notification may be defined in other PKIX
specifications. On-line methods of revocation notification may be
applicable in some environments as an alternative to the X.509 CRL.
On-line revocation checking may significantly reduce the latency
between a revocation report and the distribution of the information
to relying parties. Once the CA accepts the report as authentic and
valid, any query to the on-line service will correctly reflect the
certificate validation impacts of the revocation. However, these
methods impose new security requirements; the certificate validator
shall trust the on-line validation service while the repository does
not need to be trusted.
3.4 Operational Protocols
Operational protocols are required to deliver certificates and CRLs
(or status information) to certificate using client systems.
Provision is needed for a variety of different means of certificate
and CRL delivery, including distribution procedures based on LDAP,
HTTP, FTP, and X.500. Operational protocols supporting these
functions are defined in other PKIX specifications. These
specifications may include definitions of message formats and
procedures for supporting all of the above operational environments,
including definitions of or references to appropriate MIME content
types.
3.5 Management Protocols
Management protocols are required to support on-line interactions
between PKI user and management entities. For example, a management
protocol might be used between a CA and a client system with which a
key pair is associated, or between two CAs which cross-certify each
other. The set of functions which potentially need to be supported
by management protocols include:
(a) registration: This is the process whereby a user first makes
itself known to a CA (directly, or through an RA), prior to that
CA issuing a certificate or certificates for that user.
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RFC 2459 Internet X.509 Public Key Infrastructure January 1999
(b) initialization: Before a client system can operate securely
it is necessary to install key materials which have the
appropriate relationship with keys stored elsewhere in the
infrastructure. For example, the client needs to be securely
initialized with the public key and other assured information of
the trusted CA(s), to be used in validating certificate paths.
Furthermore, a client typically needs to be initialized with its
own key pair(s).
(c) certification: This is the process in which a CA issues a
certificate for a user's public key, and returns that certificate
to the user's client system and/or posts that certificate in a
repository.
(d) key pair recovery: As an option, user client key materials
(e.g., a user's private key used for encryption purposes) may be
backed up by a CA or a key backup system. If a user needs to
recover these backed up key materials (e.g., as a result of a
forgotten password or a lost key chain file), an on-line protocol
exchange may be needed to support such recovery.
(e) key pair update: All key pairs need to be updated regularly,
i.e., replaced with a new key pair, and new certificates issued.
(f) revocation request: An authorized person advises a CA of an
abnormal situation requiring certificate revocation.
(g) cross-certification: Two CAs exchange information used in
establishing a cross-certificate. A cross-certificate is a
certificate issued by one CA to another CA which contains a CA
signature key used for issuing certificates.
Note that on-line protocols are not the only way of implementing the
above functions. For all functions there are off-line methods of
achieving the same result, and this specification does not mandate
use of on-line protocols. For example, when hardware tokens are
used, many of the functions may be achieved as part of the physical
token delivery. Furthermore, some of the above functions may be
combined into one protocol exchange. In particular, two or more of
the registration, initialization, and certification functions can be
combined into one protocol exchange.
The PKIX series of specifications may define a set of standard
message formats supporting the above functions in future
specifications. In that case, the protocols for conveying these
messages in different environments (e.g., on-line, file transfer, e-
mail, and WWW) will also be described in those specifications.
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RFC 2459 Internet X.509 Public Key Infrastructure January 19994 Certificate and Certificate Extensions Profile
This section presents a profile for public key certificates that will
foster interoperability and a reusable PKI. This section is based
upon the X.509 v3 certificate format and the standard certificate
extensions defined in [X.509]. The ISO/IEC/ITU documents use the
1993 version of ASN.1; while this document uses the 1988 ASN.1
syntax, the encoded certificate and standard extensions are
equivalent. This section also defines private extensions required to
support a PKI for the Internet community.
Certificates may be used in a wide range of applications and
environments covering a broad spectrum of interoperability goals and
a broader spectrum of operational and assurance requirements. The
goal of this document is to establish a common baseline for generic
applications requiring broad interoperability and limited special
purpose requirements. In particular, the emphasis will be on
supporting the use of X.509 v3 certificates for informal Internet
electronic mail, IPsec, and WWW applications.
4.1 Basic Certificate Fields
The X.509 v3 certificate basic syntax is as follows. For signature
calculation, the certificate is encoded using the ASN.1 distinguished
encoding rules (DER) [X.208]. ASN.1 DER encoding is a tag, length,
value encoding system for each element.
Certificate ::= SEQUENCE {
tbsCertificate TBSCertificate,
signatureAlgorithm AlgorithmIdentifier,
signatureValue BIT STRING }
TBSCertificate ::= SEQUENCE {
version [0] EXPLICIT Version DEFAULT v1,
serialNumber CertificateSerialNumber,
signature AlgorithmIdentifier,
issuer Name,
validity Validity,
subject Name,
subjectPublicKeyInfo SubjectPublicKeyInfo,
issuerUniqueID [1] IMPLICIT UniqueIdentifier OPTIONAL,
-- If present, version shall be v2 or v3
subjectUniqueID [2] IMPLICIT UniqueIdentifier OPTIONAL,
-- If present, version shall be v2 or v3
extensions [3] EXPLICIT Extensions OPTIONAL
-- If present, version shall be v3
}
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RFC 2459 Internet X.509 Public Key Infrastructure January 1999
Version ::= INTEGER { v1(0), v2(1), v3(2) }
CertificateSerialNumber ::= INTEGER
Validity ::= SEQUENCE {
notBefore Time,
notAfter Time }
Time ::= CHOICE {
utcTime UTCTime,
generalTime GeneralizedTime }
UniqueIdentifier ::= BIT STRING
SubjectPublicKeyInfo ::= SEQUENCE {
algorithm AlgorithmIdentifier,
subjectPublicKey BIT STRING }
Extensions ::= SEQUENCE SIZE (1..MAX) OF Extension
Extension ::= SEQUENCE {
extnID OBJECT IDENTIFIER,
critical BOOLEAN DEFAULT FALSE,
extnValue OCTET STRING }
The following items describe the X.509 v3 certificate for use in the
Internet.
4.1.1 Certificate Fields
The Certificate is a SEQUENCE of three required fields. The fields
are described in detail in the following subsections.
4.1.1.1 tbsCertificate
The field contains the names of the subject and issuer, a public key
associated with the subject, a validity period, and other associated
information. The fields are described in detail in section 4.1.2;
the tbscertificate may also include extensions which are described in
section 4.2.
4.1.1.2 signatureAlgorithm
The signatureAlgorithm field contains the identifier for the
cryptographic algorithm used by the CA to sign this certificate.
Section 7.2 lists the supported signature algorithms.
An algorithm identifier is defined by the following ASN.1 structure:
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RFC 2459 Internet X.509 Public Key Infrastructure January 1999
AlgorithmIdentifier ::= SEQUENCE {
algorithm OBJECT IDENTIFIER,
parameters ANY DEFINED BY algorithm OPTIONAL }
The algorithm identifier is used to identify a cryptographic
algorithm. The OBJECT IDENTIFIER component identifies the algorithm
(such as DSA with SHA-1). The contents of the optional parameters
field will vary according to the algorithm identified. Section 7.2
lists the supported algorithms for this specification.
This field MUST contain the same algorithm identifier as the
signature field in the sequence tbsCertificate (see sec. 4.1.2.3).
4.1.1.3 signatureValue
The signatureValue field contains a digital signature computed upon
the ASN.1 DER encoded tbsCertificate. The ASN.1 DER encoded
tbsCertificate is used as the input to the signature function. This
signature value is then ASN.1 encoded as a BIT STRING and included in
the Certificate's signature field. The details of this process are
specified for each of the supported algorithms in Section 7.2.
By generating this signature, a CA certifies the validity of the
information in the tbsCertificate field. In particular, the CA
certifies the binding between the public key material and the subject
of the certificate.
4.1.2 TBSCertificate
The sequence TBSCertificate contains information associated with the
subject of the certificate and the CA who issued it. Every
TBSCertificate contains the names of the subject and issuer, a public
key associated with the subject, a validity period, a version number,
and a serial number; some may contain optional unique identifier
fields. The remainder of this section describes the syntax and
semantics of these fields. A TBSCertificate may also include
extensions. Extensions for the Internet PKI are described in Section4.2.
4.1.2.1 Version
This field describes the version of the encoded certificate. When
extensions are used, as expected in this profile, use X.509 version 3
(value is 2). If no extensions are present, but a UniqueIdentifier
is present, use version 2 (value is 1). If only basic fields are
present, use version 1 (the value is omitted from the certificate as
the default value).
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RFC 2459 Internet X.509 Public Key Infrastructure January 1999
Implementations SHOULD be prepared to accept any version certificate.
At a minimum, conforming implementations MUST recognize version 3
certificates.
Generation of version 2 certificates is not expected by
implementations based on this profile.
4.1.2.2 Serial number
The serial number is an integer assigned by the CA to each
certificate. It MUST be unique for each certificate issued by a
given CA (i.e., the issuer name and serial number identify a unique
certificate).
4.1.2.3 Signature
This field contains the algorithm identifier for the algorithm used
by the CA to sign the certificate.
This field MUST contain the same algorithm identifier as the
signatureAlgorithm field in the sequence Certificate (see sec.
4.1.1.2). The contents of the optional parameters field will vary
according to the algorithm identified. Section 7.2 lists the
supported signature algorithms.
4.1.2.4 Issuer
The issuer field identifies the entity who has signed and issued the
certificate. The issuer field MUST contain a non-empty distinguished
name (DN). The issuer field is defined as the X.501 type Name.
[X.501] Name is defined by the following ASN.1 structures:
Name ::= CHOICE {
RDNSequence }
RDNSequence ::= SEQUENCE OF RelativeDistinguishedName
RelativeDistinguishedName ::=
SET OF AttributeTypeAndValue
AttributeTypeAndValue ::= SEQUENCE {
type AttributeType,
value AttributeValue }
AttributeType ::= OBJECT IDENTIFIER
AttributeValue ::= ANY DEFINED BY AttributeType
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RFC 2459 Internet X.509 Public Key Infrastructure January 1999
DirectoryString ::= CHOICE {
teletexString TeletexString (SIZE (1..MAX)),
printableString PrintableString (SIZE (1..MAX)),
universalString UniversalString (SIZE (1..MAX)),
utf8String UTF8String (SIZE (1.. MAX)),
bmpString BMPString (SIZE (1..MAX)) }
The Name describes a hierarchical name composed of attributes, such
as country name, and corresponding values, such as US. The type of
the component AttributeValue is determined by the AttributeType; in
general it will be a DirectoryString.
The DirectoryString type is defined as a choice of PrintableString,
TeletexString, BMPString, UTF8String, and UniversalString. The
UTF8String encoding is the preferred encoding, and all certificates
issued after December 31, 2003 MUST use the UTF8String encoding of
DirectoryString (except as noted below). Until that date, conforming
CAs MUST choose from the following options when creating a
distinguished name, including their own:
(a) if the character set is sufficient, the string MAY be
represented as a PrintableString;
(b) failing (a), if the BMPString character set is sufficient the
string MAY be represented as a BMPString; and
(c) failing (a) and (b), the string MUST be represented as a
UTF8String. If (a) or (b) is satisfied, the CA MAY still choose
to represent the string as a UTF8String.
Exceptions to the December 31, 2003 UTF8 encoding requirements are as
follows:
(a) CAs MAY issue "name rollover" certificates to support an
orderly migration to UTF8String encoding. Such certificates would
include the CA's UTF8String encoded name as issuer and and the old
name encoding as subject, or vice-versa.
(b) As stated in section 4.1.2.6, the subject field MUST be
populated with a non-empty distinguished name matching the
contents of the issuer field in all certificates issued by the
subject CA regardless of encoding.
The TeletexString and UniversalString are included for backward
compatibility, and should not be used for certificates for new
subjects. However, these types may be used in certificates where the
name was previously established. Certificate users SHOULD be
prepared to receive certificates with these types.
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RFC 2459 Internet X.509 Public Key Infrastructure January 1999
In addition, many legacy implementations support names encoded in the
ISO 8859-1 character set (Latin1String) but tag them as
TeletexString. The Latin1String includes characters used in Western
European countries which are not part of the TeletexString charcter
set. Implementations that process TeletexString SHOULD be prepared
to handle the entire ISO 8859-1 character set.[ISO 8859-1]
As noted above, distinguished names are composed of attributes. This
specification does not restrict the set of attribute types that may
appear in names. However, conforming implementations MUST be
prepared to receive certificates with issuer names containing the set
of attribute types defined below. This specification also recommends
support for additional attribute types.
Standard sets of attributes have been defined in the X.500 series of
specifications.[X.520] Implementations of this specification MUST be
prepared to receive the following standard attribute types in issuer
names: country, organization, organizational-unit, distinguished name
qualifier, state or province name, and common name (e.g., "Susan
Housley"). In addition, implementations of this specification SHOULD
be prepared to receive the following standard attribute types in
issuer names: locality, title, surname, given name, initials, and
generation qualifier (e.g., "Jr.", "3rd", or "IV"). The syntax and
associated object identifiers (OIDs) for these attribute types are
provided in the ASN.1 modules in Appendices A and B.
In addition, implementations of this specification MUST be prepared
to receive the domainComponent attribute, as defined in [RFC 2247].
The Domain (Nameserver) System (DNS) provides a hierarchical resource
labeling system. This attribute provides is a convenient mechanism
for organizations that wish to use DNs that parallel their DNS names.
This is not a replacement for the dNSName component of the
alternative name field. Implementations are not required to convert
such names into DNS names. The syntax and associated OID for this
attribute type is provided in the ASN.1 modules in Appendices A and
B.
Certificate users MUST be prepared to process the issuer
distinguished name and subject distinguished name (see sec. 4.1.2.6)
fields to perform name chaining for certification path validation
(see section 6). Name chaining is performed by matching the issuer
distinguished name in one certificate with the subject name in a CA
certificate.
This specification requires only a subset of the name comparison
functionality specified in the X.500 series of specifications. The
requirements for conforming implementations are as follows:
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RFC 2459 Internet X.509 Public Key Infrastructure January 1999
(a) attribute values encoded in different types (e.g.,
PrintableString and BMPString) may be assumed to represent
different strings;
(b) attribute values in types other than PrintableString are case
sensitive (this permits matching of attribute values as binary
objects);
(c) attribute values in PrintableString are not case sensitive
(e.g., "Marianne Swanson" is the same as "MARIANNE SWANSON"); and
(d) attribute values in PrintableString are compared after
removing leading and trailing white space and converting internal
substrings of one or more consecutive white space characters to a
single space.
These name comparison rules permit a certificate user to validate
certificates issued using languages or encodings unfamiliar to the
certificate user.
In addition, implementations of this specification MAY use these
comparison rules to process unfamiliar attribute types for name
chaining. This allows implementations to process certificates with
unfamiliar attributes in the issuer name.
Note that the comparison rules defined in the X.500 series of
specifications indicate that the character sets used to encode data
in distinguished names are irrelevant. The characters themselves are
compared without regard to encoding. Implementations of the profile
are permitted to use the comparison algorithm defined in the X.500
series. Such an implementation will recognize a superset of name
matches recognized by the algorithm specified above.
4.1.2.5 Validity
The certificate validity period is the time interval during which the
CA warrants that it will maintain information about the status of the
certificate. The field is represented as a SEQUENCE of two dates:
the date on which the certificate validity period begins (notBefore)
and the date on which the certificate validity period ends
(notAfter). Both notBefore and notAfter may be encoded as UTCTime or
GeneralizedTime.
CAs conforming to this profile MUST always encode certificate
validity dates through the year 2049 as UTCTime; certificate validity
dates in 2050 or later MUST be encoded as GeneralizedTime.
Housley, et. al. Standards Track [Page 21]

RFC 2459 Internet X.509 Public Key Infrastructure January 19994.1.2.5.1 UTCTime
The universal time type, UTCTime, is a standard ASN.1 type intended
for international applications where local time alone is not
adequate. UTCTime specifies the year through the two low order
digits and time is specified to the precision of one minute or one
second. UTCTime includes either Z (for Zulu, or Greenwich Mean Time)
or a time differential.
For the purposes of this profile, UTCTime values MUST be expressed
Greenwich Mean Time (Zulu) and MUST include seconds (i.e., times are
YYMMDDHHMMSSZ), even where the number of seconds is zero. Conforming
systems MUST interpret the year field (YY) as follows:
Where YY is greater than or equal to 50, the year shall be
interpreted as 19YY; and
Where YY is less than 50, the year shall be interpreted as 20YY.
4.1.2.5.2 GeneralizedTime
The generalized time type, GeneralizedTime, is a standard ASN.1 type
for variable precision representation of time. Optionally, the
GeneralizedTime field can include a representation of the time
differential between local and Greenwich Mean Time.
For the purposes of this profile, GeneralizedTime values MUST be
expressed Greenwich Mean Time (Zulu) and MUST include seconds (i.e.,
times are YYYYMMDDHHMMSSZ), even where the number of seconds is zero.
GeneralizedTime values MUST NOT include fractional seconds.
4.1.2.6 Subject
The subject field identifies the entity associated with the public
key stored in the subject public key field. The subject name may be
carried in the subject field and/or the subjectAltName extension. If
the subject is a CA (e.g., the basic constraints extension, as
discussed in 4.2.1.10, is present and the value of cA is TRUE,) then
the subject field MUST be populated with a non-empty distinguished
name matching the contents of the issuer field (see sec. 4.1.2.4) in
all certificates issued by the subject CA. If subject naming
information is present only in the subjectAltName extension (e.g., a
key bound only to an email address or URI), then the subject name
MUST be an empty sequence and the subjectAltName extension MUST be
critical.
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RFC 2459 Internet X.509 Public Key Infrastructure January 1999
Where it is non-empty, the subject field MUST contain an X.500
distinguished name (DN). The DN MUST be unique for each subject
entity certified by the one CA as defined by the issuer name field. A
CA may issue more than one certificate with the same DN to the same
subject entity.
The subject name field is defined as the X.501 type Name.
Implementation requirements for this field are those defined for the
issuer field (see sec. 4.1.2.4). When encoding attribute values of
type DirectoryString, the encoding rules for the issuer field MUST be
implemented. Implementations of this specification MUST be prepared
to receive subject names containing the attribute types required for
the issuer field. Implementations of this specification SHOULD be
prepared to receive subject names containing the recommended
attribute types for the issuer field. The syntax and associated
object identifiers (OIDs) for these attribute types are provided in
the ASN.1 modules in Appendices A and B. Implementations of this
specification MAY use these comparison rules to process unfamiliar
attribute types (i.e., for name chaining). This allows
implementations to process certificates with unfamiliar attributes in
the subject name.
In addition, legacy implementations exist where an RFC 822 name is
embedded in the subject distinguished name as an EmailAddress
attribute. The attribute value for EmailAddress is of type IA5String
to permit inclusion of the character '@', which is not part of the
PrintableString character set. EmailAddress attribute values are not
case sensitive (e.g., "fanfeedback@redsox.com" is the same as
"FANFEEDBACK@REDSOX.COM").
Conforming implementations generating new certificates with
electronic mail addresses MUST use the rfc822Name in the subject
alternative name field (see sec. 4.2.1.7) to describe such
identities. Simultaneous inclusion of the EmailAddress attribute in
the subject distinguished name to support legacy implementations is
deprecated but permitted.
4.1.2.7 Subject Public Key Info
This field is used to carry the public key and identify the algorithm
with which the key is used. The algorithm is identified using the
AlgorithmIdentifier structure specified in section 4.1.1.2. The
object identifiers for the supported algorithms and the methods for
encoding the public key materials (public key and parameters) are
specified in section 7.3.
Housley, et. al. Standards Track [Page 23]

RFC 2459 Internet X.509 Public Key Infrastructure January 19994.1.2.8 Unique Identifiers
These fields may only appear if the version is 2 or 3 (see sec.
4.1.2.1). The subject and issuer unique identifiers are present in
the certificate to handle the possibility of reuse of subject and/or
issuer names over time. This profile recommends that names not be
reused for different entities and that Internet certificates not make
use of unique identifiers. CAs conforming to this profile SHOULD NOT
generate certificates with unique identifiers. Applications
conforming to this profile SHOULD be capable of parsing unique
identifiers and making comparisons.
4.1.2.9 Extensions
This field may only appear if the version is 3 (see sec. 4.1.2.1).
If present, this field is a SEQUENCE of one or more certificate
extensions. The format and content of certificate extensions in the
Internet PKI is defined in section 4.2.
4.2 Standard Certificate Extensions
The extensions defined for X.509 v3 certificates provide methods for
associating additional attributes with users or public keys and for
managing the certification hierarchy. The X.509 v3 certificate
format also allows communities to define private extensions to carry
information unique to those communities. Each extension in a
certificate may be designated as critical or non-critical. A
certificate using system MUST reject the certificate if it encounters
a critical extension it does not recognize; however, a non-critical
extension may be ignored if it is not recognized. The following
sections present recommended extensions used within Internet
certificates and standard locations for information. Communities may
elect to use additional extensions; however, caution should be
exercised in adopting any critical extensions in certificates which
might prevent use in a general context.
Each extension includes an OID and an ASN.1 structure. When an
extension appears in a certificate, the OID appears as the field
extnID and the corresponding ASN.1 encoded structure is the value of
the octet string extnValue. Only one instance of a particular
extension may appear in a particular certificate. For example, a
certificate may contain only one authority key identifier extension
(see sec. 4.2.1.1). An extension includes the boolean critical, with
a default value of FALSE. The text for each extension specifies the
acceptable values for the critical field.
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RFC 2459 Internet X.509 Public Key Infrastructure January 1999
Conforming CAs MUST support key identifiers (see sec. 4.2.1.1 and
4.2.1.2), basic constraints (see sec. 4.2.1.10), key usage (see sec.
4.2.1.3), and certificate policies (see sec. 4.2.1.5) extensions. If
the CA issues certificates with an empty sequence for the subject
field, the CA MUST support the subject alternative name extension
(see sec. 4.2.1.7). Support for the remaining extensions is
OPTIONAL. Conforming CAs may support extensions that are not
identified within this specification; certificate issuers are
cautioned that marking such extensions as critical may inhibit
interoperability.
At a minimum, applications conforming to this profile MUST recognize
the extensions which must or may be critical in this specification.
These extensions are: key usage (see sec. 4.2.1.3), certificate
policies (see sec. 4.2.1.5), the subject alternative name (see sec.
4.2.1.7), basic constraints (see sec. 4.2.1.10), name constraints
(see sec. 4.2.1.11), policy constraints (see sec. 4.2.1.12), and
extended key usage (see sec. 4.2.1.13).
In addition, this profile RECOMMENDS application support for the
authority and subject key identifier (see sec. 4.2.1.1 and 4.2.1.2)
extensions.
4.2.1 Standard Extensions
This section identifies standard certificate extensions defined in
[X.509] for use in the Internet PKI. Each extension is associated
with an OID defined in [X.509]. These OIDs are members of the id-ce
arc, which is defined by the following:
id-ce OBJECT IDENTIFIER ::= {joint-iso-ccitt(2) ds(5) 29}
4.2.1.1 Authority Key Identifier
The authority key identifier extension provides a means of
identifying the public key corresponding to the private key used to
sign a certificate. This extension is used where an issuer has
multiple signing keys (either due to multiple concurrent key pairs or
due to changeover). The identification may be based on either the
key identifier (the subject key identifier in the issuer's
certificate) or on the issuer name and serial number.
The keyIdentifier field of the authorityKeyIdentifier extension MUST
be included in all certificates generated by conforming CAs to
facilitate chain building. There is one exception; where a CA
distributes its public key in the form of a "self-signed"
certificate, the authority key identifier may be omitted. In this
case, the subject and authority key identifiers would be identical.
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RFC 2459 Internet X.509 Public Key Infrastructure January 1999
The value of the keyIdentifier field SHOULD be derived from the
public key used to verify the certificate's signature or a method
that generates unique values. Two common methods for generating key
identifiers from the public key are described in (sec. 4.2.1.2). One
common method for generating unique values isdescribed in (sec.
4.2.1.2). Where a key identifier has not been previously
established, this specification recommends use of one of these
methods for generating keyIdentifiers.
This profile recommends support for the key identifier method by all
certificate users.
This extension MUST NOT be marked critical.
id-ce-authorityKeyIdentifier OBJECT IDENTIFIER ::= { id-ce 35 }
AuthorityKeyIdentifier ::= SEQUENCE {
keyIdentifier [0] KeyIdentifier OPTIONAL,
authorityCertIssuer [1] GeneralNames OPTIONAL,
authorityCertSerialNumber [2] CertificateSerialNumber OPTIONAL }
KeyIdentifier ::= OCTET STRING
4.2.1.2 Subject Key Identifier
The subject key identifier extension provides a means of identifying
certificates that contain a particular public key.
To facilitate chain building, this extension MUST appear in all con-
forming CA certificates, that is, all certificates including the
basic constraints extension (see sec. 4.2.1.10) where the value of cA
is TRUE. The value of the subject key identifier MUST be the value
placed in the key identifier field of the Authority Key Identifier
extension (see sec. 4.2.1.1) of certificates issued by the subject of
this certificate.
For CA certificates, subject key identifiers SHOULD be derived from
the public key or a method that generates unique values. Two common
methods for generating key identifiers from the public key are:
(1) The keyIdentifier is composed of the 160-bit SHA-1 hash of the
value of the BIT STRING subjectPublicKey (excluding the tag,
length, and number of unused bits).
(2) The keyIdentifier is composed of a four bit type field with
the value 0100 followed by the least significant 60 bits of the
SHA-1 hash of the value of the BIT STRING subjectPublicKey.
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RFC 2459 Internet X.509 Public Key Infrastructure January 1999
One common method for generating unique values is a monotomically
increasing sequence of integers.
For end entity certificates, the subject key identifier extension
provides a means for identifying certificates containing the
particular public key used in an application. Where an end entity has
obtained multiple certificates, especially from multiple CAs, the
subject key identifier provides a means to quickly identify the set
of certificates containing a particular public key. To assist
applications in identificiation the appropriate end entity
certificate, this extension SHOULD be included in all end entity
certificates.
For end entity certificates, subject key identifiers SHOULD be
derived from the public key. Two common methods for generating key
identifiers from the public key are identifed above.
Where a key identifier has not been previously established, this
specification recommends use of one of these methods for generating
keyIdentifiers.
This extension MUST NOT be marked critical.
id-ce-subjectKeyIdentifier OBJECT IDENTIFIER ::= { id-ce 14 }
SubjectKeyIdentifier ::= KeyIdentifier
4.2.1.3 Key Usage
The key usage extension defines the purpose (e.g., encipherment,
signature, certificate signing) of the key contained in the
certificate. The usage restriction might be employed when a key that
could be used for more than one operation is to be restricted. For
example, when an RSA key should be used only for signing, the
digitalSignature and/or nonRepudiation bits would be asserted.
Likewise, when an RSA key should be used only for key management, the
keyEncipherment bit would be asserted. When used, this extension
SHOULD be marked critical.
id-ce-keyUsage OBJECT IDENTIFIER ::= { id-ce 15 }
KeyUsage ::= BIT STRING {
digitalSignature (0),
nonRepudiation (1),
keyEncipherment (2),
dataEncipherment (3),
keyAgreement (4),
keyCertSign (5),
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RFC 2459 Internet X.509 Public Key Infrastructure January 1999
cRLSign (6),
encipherOnly (7),
decipherOnly (8) }
Bits in the KeyUsage type are used as follows:
The digitalSignature bit is asserted when the subject public key
is used with a digital signature mechanism to support security
services other than non-repudiation (bit 1), certificate signing
(bit 5), or revocation information signing (bit 6). Digital
signature mechanisms are often used for entity authentication and
data origin authentication with integrity.
The nonRepudiation bit is asserted when the subject public key is
used to verify digital signatures used to provide a non-
repudiation service which protects against the signing entity
falsely denying some action, excluding certificate or CRL signing.
The keyEncipherment bit is asserted when the subject public key is
used for key transport. For example, when an RSA key is to be
used for key management, then this bit shall asserted.
The dataEncipherment bit is asserted when the subject public key
is used for enciphering user data, other than cryptographic keys.
The keyAgreement bit is asserted when the subject public key is
used for key agreement. For example, when a Diffie-Hellman key is
to be used for key management, then this bit shall asserted.
The keyCertSign bit is asserted when the subject public key is
used for verifying a signature on certificates. This bit may only
be asserted in CA certificates.
The cRLSign bit is asserted when the subject public key is used
for verifying a signature on revocation information (e.g., a CRL).
The meaning of the encipherOnly bit is undefined in the absence of
the keyAgreement bit. When the encipherOnly bit is asserted and
the keyAgreement bit is also set, the subject public key may be
used only for enciphering data while performing key agreement.
The meaning of the decipherOnly bit is undefined in the absence of
the keyAgreement bit. When the decipherOnly bit is asserted and
the keyAgreement bit is also set, the subject public key may be
used only for deciphering data while performing key agreement.
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RFC 2459 Internet X.509 Public Key Infrastructure January 1999
This profile does not restrict the combinations of bits that may be
set in an instantiation of the keyUsage extension. However,
appropriate values for keyUsage extensions for particular algorithms
are specified in section 7.3.
4.2.1.4 Private Key Usage Period
This profile recommends against the use of this extension. CAs
conforming to this profile MUST NOT generate certificates with
critical private key usage period extensions.
The private key usage period extension allows the certificate issuer
to specify a different validity period for the private key than the
certificate. This extension is intended for use with digital
signature keys. This extension consists of two optional components,
notBefore and notAfter. The private key associated with the
certificate should not be used to sign objects before or after the
times specified by the two components, respectively. CAs conforming
to this profile MUST NOT generate certificates with private key usage
period extensions unless at least one of the two components is
present.
Where used, notBefore and notAfter are represented as GeneralizedTime
and MUST be specified and interpreted as defined in section4.1.2.5.2.
id-ce-privateKeyUsagePeriod OBJECT IDENTIFIER ::= { id-ce 16 }
PrivateKeyUsagePeriod ::= SEQUENCE {
notBefore [0] GeneralizedTime OPTIONAL,
notAfter [1] GeneralizedTime OPTIONAL }
4.2.1.5 Certificate Policies
The certificate policies extension contains a sequence of one or more
policy information terms, each of which consists of an object
identifier (OID) and optional qualifiers. These policy information
terms indicate the policy under which the certificate has been issued
and the purposes for which the certificate may be used. Optional
qualifiers, which may be present, are not expected to change the
definition of the policy.
Applications with specific policy requirements are expected to have a
list of those policies which they will accept and to compare the
policy OIDs in the certificate to that list. If this extension is
critical, the path validation software MUST be able to interpret this
extension (including the optional qualifier), or MUST reject the
certificate.
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RFC 2459 Internet X.509 Public Key Infrastructure January 1999
To promote interoperability, this profile RECOMMENDS that policy
information terms consist of only an OID. Where an OID alone is
insufficient, this profile strongly recommends that use of qualifiers
be limited to those identified in this section.
This specification defines two policy qualifier types for use by
certificate policy writers and certificate issuers. The qualifier
types are the CPS Pointer and User Notice qualifiers.
The CPS Pointer qualifier contains a pointer to a Certification
Practice Statement (CPS) published by the CA. The pointer is in the
form of a URI.
User notice is intended for display to a relying party when a
certificate is used. The application software SHOULD display all
user notices in all certificates of the certification path used,
except that if a notice is duplicated only one copy need be
displayed. To prevent such duplication, this qualifier SHOULD only
be present in end-entity certificates and CA certificates issued to
other organizations.
The user notice has two optional fields: the noticeRef field and the
explicitText field.
The noticeRef field, if used, names an organization and
identifies, by number, a particular textual statement prepared by
that organization. For example, it might identify the
organization "CertsRUs" and notice number 1. In a typical
implementation, the application software will have a notice file
containing the current set of notices for CertsRUs; the
application will extract the notice text from the file and display
it. Messages may be multilingual, allowing the software to select
the particular language message for its own environment.
An explicitText field includes the textual statement directly in
the certificate. The explicitText field is a string with a
maximum size of 200 characters.
If both the noticeRef and explicitText options are included in the
one qualifier and if the application software can locate the notice
text indicated by the noticeRef option then that text should be
displayed; otherwise, the explicitText string should be displayed.
id-ce-certificatePolicies OBJECT IDENTIFIER ::= { id-ce 32 }
certificatePolicies ::= SEQUENCE SIZE (1..MAX) OF PolicyInformation
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RFC 2459 Internet X.509 Public Key Infrastructure January 1999
The issuing CA's users may accept an issuerDomainPolicy for certain
applications. The policy mapping tells the issuing CA's users which
policies associated with the subject CA are comparable to the policy
they accept.
This extension may be supported by CAs and/or applications, and it
MUST be non-critical.
id-ce-policyMappings OBJECT IDENTIFIER ::= { id-ce 33 }
PolicyMappings ::= SEQUENCE SIZE (1..MAX) OF SEQUENCE {
issuerDomainPolicy CertPolicyId,
subjectDomainPolicy CertPolicyId }
4.2.1.7 Subject Alternative Name
The subject alternative names extension allows additional identities
to be bound to the subject of the certificate. Defined options
include an Internet electronic mail address, a DNS name, an IP
address, and a uniform resource identifier (URI). Other options
exist, including completely local definitions. Multiple name forms,
and multiple instances of each name form, may be included. Whenever
such identities are to be bound into a certificate, the subject
alternative name (or issuer alternative name) extension MUST be used.
Because the subject alternative name is considered to be
definitiviely bound to the public key, all parts of the subject
alternative name MUST be verified by the CA.
Further, if the only subject identity included in the certificate is
an alternative name form (e.g., an electronic mail address), then the
subject distinguished name MUST be empty (an empty sequence), and the
subjectAltName extension MUST be present. If the subject field
contains an empty sequence, the subjectAltName extension MUST be
marked critical.
When the subjectAltName extension contains an Internet mail address,
the address MUST be included as an rfc822Name. The format of an
rfc822Name is an "addr-spec" as defined in RFC 822 [RFC 822]. An
addr-spec has the form "local-part@domain". Note that an addr-spec
has no phrase (such as a common name) before it, has no comment (text
surrounded in parentheses) after it, and is not surrounded by "<" and
">". Note that while upper and lower case letters are allowed in an
RFC 822 addr-spec, no significance is attached to the case.
When the subjectAltName extension contains a iPAddress, the address
MUST be stored in the octet string in "network byte order," as
specified in RFC 791 [RFC 791]. The least significant bit (LSB) of
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RFC 2459 Internet X.509 Public Key Infrastructure January 1999
each octet is the LSB of the corresponding byte in the network
address. For IP Version 4, as specified in RFC 791, the octet string
MUST contain exactly four octets. For IP Version 6, as specified in
RFC 1883, the octet string MUST contain exactly sixteen octets [RFC
1883].
When the subjectAltName extension contains a domain name service
label, the domain name MUST be stored in the dNSName (an IA5String).
The name MUST be in the "preferred name syntax," as specified by RFC1034 [RFC 1034]. Note that while upper and lower case letters are
allowed in domain names, no signifigance is attached to the case. In
addition, while the string " " is a legal domain name, subjectAltName
extensions with a dNSName " " are not permitted. Finally, the use of
the DNS representation for Internet mail addresses (wpolk.nist.gov
instead of wpolk@nist.gov) is not permitted; such identities are to
be encoded as rfc822Name.
When the subjectAltName extension contains a URI, the name MUST be
stored in the uniformResourceIdentifier (an IA5String). The name MUST
be a non-relative URL, and MUST follow the URL syntax and encoding
rules specified in [RFC 1738]. The name must include both a scheme
(e.g., "http" or "ftp") and a scheme-specific-part. The scheme-
specific-part must include a fully qualified domain name or IP
address as the host.
As specified in [RFC 1738], the scheme name is not case-sensitive
(e.g., "http" is equivalent to "HTTP"). The host part is also not
case-sensitive, but other components of the scheme-specific-part may
be case-sensitive. When comparing URIs, conforming implementations
MUST compare the scheme and host without regard to case, but assume
the remainder of the scheme-specific-part is case sensitive.
Subject alternative names may be constrained in the same manner as
subject distinguished names using the name constraints extension as
described in section 4.2.1.11.
If the subjectAltName extension is present, the sequence MUST contain
at least one entry. Unlike the subject field, conforming CAs MUST
NOT issue certificates with subjectAltNames containing empty
GeneralName fields. For example, an rfc822Name is represented as an
IA5String. While an empty string is a valid IA5String, such an
rfc822Name is not permitted by this profile. The behavior of clients
that encounter such a certificate when processing a certificication
path is not defined by this profile.
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RFC 2459 Internet X.509 Public Key Infrastructure January 1999
id-ce-subjectDirectoryAttributes OBJECT IDENTIFIER ::= { id-ce 9 }
SubjectDirectoryAttributes ::= SEQUENCE SIZE (1..MAX) OF Attribute
4.2.1.10 Basic Constraints
The basic constraints extension identifies whether the subject of the
certificate is a CA and how deep a certification path may exist
through that CA.
The pathLenConstraint field is meaningful only if cA is set to TRUE.
In this case, it gives the maximum number of CA certificates that may
follow this certificate in a certification path. A value of zero
indicates that only an end-entity certificate may follow in the path.
Where it appears, the pathLenConstraint field MUST be greater than or
equal to zero. Where pathLenConstraint does not appear, there is no
limit to the allowed length of the certification path.
This extension MUST appear as a critical extension in all CA
certificates. This extension SHOULD NOT appear in end entity
certificates.
id-ce-basicConstraints OBJECT IDENTIFIER ::= { id-ce 19 }
BasicConstraints ::= SEQUENCE {
cA BOOLEAN DEFAULT FALSE,
pathLenConstraint INTEGER (0..MAX) OPTIONAL }
4.2.1.11 Name Constraints
The name constraints extension, which MUST be used only in a CA
certificate, indicates a name space within which all subject names in
subsequent certificates in a certification path shall be located.
Restrictions may apply to the subject distinguished name or subject
alternative names. Restrictions apply only when the specified name
form is present. If no name of the type is in the certificate, the
certificate is acceptable.
Restrictions are defined in terms of permitted or excluded name
subtrees. Any name matching a restriction in the excludedSubtrees
field is invalid regardless of information appearing in the
permittedSubtrees. This extension MUST be critical.
Within this profile, the minimum and maximum fields are not used with
any name forms, thus minimum is always zero, and maximum is always
absent.
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RFC 2459 Internet X.509 Public Key Infrastructure January 1999
For URIs, the constraint applies to the host part of the name. The
constraint may specify a host or a domain. Examples would be
"foo.bar.com"; and ".xyz.com". When the the constraint begins with
a period, it may be expanded with one or more subdomains. That is,
the constraint ".xyz.com" is satisfied by both abc.xyz.com and
abc.def.xyz.com. However, the constraint ".xyz.com" is not satisfied
by "xyz.com". When the constraint does not begin with a period, it
specifies a host.
A name constraint for Internat mail addresses may specify a
particular mailbox, all addresses at a particular host, or all
mailboxes in a domain. To indicate a particular mailbox, the
constraint is the complete mail address. For example, "root@xyz.com"
indicates the root mailbox on the host "xyz.com". To indicate all
Internet mail addresses on a particular host, the constraint is
specified as the host name. For example, the constraint "xyz.com" is
satisfied by any mail address at the host "xyz.com". To specify any
address within a domain, the constraint is specified with a leading
period (as with URIs). For example, ".xyz.com" indicates all the
Internet mail addresses in the domain "xyz.com", but Internet mail
addresses on the host "xyz.com".
DNS name restrictions are expressed as foo.bar.com. Any subdomain
satisfies the name constraint. For example, www.foo.bar.com would
satisfy the constraint but bigfoo.bar.com would not.
Legacy implementations exist where an RFC 822 name is embedded in the
subject distinguished name in an attribute of type EmailAddress (see
sec. 4.1.2.6). When rfc822 names are constrained, but the certificate
does not include a subject alternative name, the rfc822 name
constraint MUST be applied to the attribute of type EmailAddress in
the subject distinguished name. The ASN.1 syntax for EmailAddress
and the corresponding OID are supplied in Appendix A and B.
Restrictions of the form directoryName MUST be applied to the subject
field in the certificate and to the subjectAltName extensions of type
directoryName. Restrictions of the form x400Address MUST be applied
to subjectAltName extensions of type x400Address.
When applying restrictions of the form directoryName, an
implementation MUST compare DN attributes. At a minimum,
implementations MUST perform the DN comparison rules specified in
Section 4.1.2.4. CAs issuing certificates with a restriction of the
form directoryName SHOULD NOT rely on implementation of the full ISO
DN name comparison algorithm. This implies name restrictions shall
be stated identically to the encoding used in the subject field or
subjectAltName extension.
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RFC 2459 Internet X.509 Public Key Infrastructure January 1999
The syntax of iPAddress MUST be as described in section 4.2.1.7 with
the following additions specifically for Name Constraints. For IPv4
addresses, the ipAddress field of generalName MUST contain eight (8)
octets, encoded in the style of RFC 1519 (CIDR) to represent an
address range.[RFC 1519] For IPv6 addresses, the ipAddress field
MUST contain 32 octets similarly encoded. For example, a name
constraint for "class C" subnet 10.9.8.0 shall be represented as the
octets 0A 09 08 00 FF FF FF 00, representing the CIDR notation
10.9.8.0/255.255.255.0.
The syntax and semantics for name constraints for otherName,
ediPartyName, and registeredID are not defined by this specification.
id-ce-nameConstraints OBJECT IDENTIFIER ::= { id-ce 30 }
NameConstraints ::= SEQUENCE {
permittedSubtrees [0] GeneralSubtrees OPTIONAL,
excludedSubtrees [1] GeneralSubtrees OPTIONAL }
GeneralSubtrees ::= SEQUENCE SIZE (1..MAX) OF GeneralSubtree
GeneralSubtree ::= SEQUENCE {
base GeneralName,
minimum [0] BaseDistance DEFAULT 0,
maximum [1] BaseDistance OPTIONAL }
BaseDistance ::= INTEGER (0..MAX)
4.2.1.12 Policy Constraints
The policy constraints extension can be used in certificates issued
to CAs. The policy constraints extension constrains path validation
in two ways. It can be used to prohibit policy mapping or require
that each certificate in a path contain an acceptable policy
identifier.
If the inhibitPolicyMapping field is present, the value indicates the
number of additional certificates that may appear in the path before
policy mapping is no longer permitted. For example, a value of one
indicates that policy mapping may be processed in certificates issued
by the subject of this certificate, but not in additional
certificates in the path.
If the requireExplicitPolicy field is present, subsequent
certificates shall include an acceptable policy identifier. The value
of requireExplicitPolicy indicates the number of additional
certificates that may appear in the path before an explicit policy is
required. An acceptable policy identifier is the identifier of a
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RFC 2459 Internet X.509 Public Key Infrastructure January 1999
policy required by the user of the certification path or the
identifier of a policy which has been declared equivalent through
policy mapping.
Conforming CAs MUST NOT issue certificates where policy constraints
is a null sequence. That is, at least one of the inhibitPolicyMapping
field or the requireExplicitPolicy field MUST be present. The
behavior of clients that encounter a null policy constraints field is
not addressed in this profile.
This extension may be critical or non-critical.
id-ce-policyConstraints OBJECT IDENTIFIER ::= { id-ce 36 }
PolicyConstraints ::= SEQUENCE {
requireExplicitPolicy [0] SkipCerts OPTIONAL,
inhibitPolicyMapping [1] SkipCerts OPTIONAL }
SkipCerts ::= INTEGER (0..MAX)
4.2.1.13 Extended key usage field
This field indicates one or more purposes for which the certified
public key may be used, in addition to or in place of the basic
purposes indicated in the key usage extension field. This field is
defined as follows:
id-ce-extKeyUsage OBJECT IDENTIFIER ::= {id-ce 37}
ExtKeyUsageSyntax ::= SEQUENCE SIZE (1..MAX) OF KeyPurposeId
KeyPurposeId ::= OBJECT IDENTIFIER
Key purposes may be defined by any organization with a need. Object
identifiers used to identify key purposes shall be assigned in
accordance with IANA or ITU-T Rec. X.660 | ISO/IEC/ITU 9834-1.
This extension may, at the option of the certificate issuer, be
either critical or non-critical.
If the extension is flagged critical, then the certificate MUST be
used only for one of the purposes indicated.
If the extension is flagged non-critical, then it indicates the
intended purpose or purposes of the key, and may be used in finding
the correct key/certificate of an entity that has multiple
keys/certificates. It is an advisory field and does not imply that
usage of the key is restricted by the certification authority to the
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RFC 2459 Internet X.509 Public Key Infrastructure January 1999
purpose indicated. Certificate using applications may nevertheless
require that a particular purpose be indicated in order for the
certificate to be acceptable to that application.
If a certificate contains both a critical key usage field and a
critical extended key usage field, then both fields MUST be processed
independently and the certificate MUST only be used for a purpose
consistent with both fields. If there is no purpose consistent with
both fields, then the certificate MUST NOT be used for any purpose.
The following key usage purposes are defined by this profile:
id-kp OBJECT IDENTIFIER ::= { id-pkix 3 }
id-kp-serverAuth OBJECT IDENTIFIER ::= {id-kp 1}
-- TLS Web server authentication
-- Key usage bits that may be consistent: digitalSignature,
-- keyEncipherment or keyAgreement
--
id-kp-clientAuth OBJECT IDENTIFIER ::= {id-kp 2}
-- TLS Web client authentication
-- Key usage bits that may be consistent: digitalSignature and/or
-- keyAgreement
--
id-kp-codeSigning OBJECT IDENTIFIER ::= {id-kp 3}
-- Signing of downloadable executable code
-- Key usage bits that may be consistent: digitalSignature
--
id-kp-emailProtection OBJECT IDENTIFIER ::= {id-kp 4}
-- E-mail protection
-- Key usage bits that may be consistent: digitalSignature,
-- nonRepudiation, and/or (keyEncipherment
-- or keyAgreement)
--
id-kp-timeStamping OBJECT IDENTIFIER ::= { id-kp 8 }
-- Binding the hash of an object to a time from an agreed-upon time
-- source. Key usage bits that may be consistent: digitalSignature,
-- nonRepudiation
4.2.1.14 CRL Distribution Points
The CRL distribution points extension identifies how CRL information
is obtained. The extension SHOULD be non-critical, but this profile
recommends support for this extension by CAs and applications.
Further discussion of CRL management is contained in section 5.
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RFC 2459 Internet X.509 Public Key Infrastructure January 1999
If the cRLDistributionPoints extension contains a
DistributionPointName of type URI, the following semantics MUST be
assumed: the URI is a pointer to the current CRL for the associated
reasons and will be issued by the associated cRLIssuer. The expected
values for the URI are those defined in 4.2.1.7. Processing rules for
other values are not defined by this specification. If the
distributionPoint omits reasons, the CRL MUST include revocations for
all reasons. If the distributionPoint omits cRLIssuer, the CRL MUST
be issued by the CA that issued the certificate.
id-ce-cRLDistributionPoints OBJECT IDENTIFIER ::= { id-ce 31 }
cRLDistributionPoints ::= {
CRLDistPointsSyntax }
CRLDistPointsSyntax ::= SEQUENCE SIZE (1..MAX) OF DistributionPoint
DistributionPoint ::= SEQUENCE {
distributionPoint [0] DistributionPointName OPTIONAL,
reasons [1] ReasonFlags OPTIONAL,
cRLIssuer [2] GeneralNames OPTIONAL }
DistributionPointName ::= CHOICE {
fullName [0] GeneralNames,
nameRelativeToCRLIssuer [1] RelativeDistinguishedName }
ReasonFlags ::= BIT STRING {
unused (0),
keyCompromise (1),
cACompromise (2),
affiliationChanged (3),
superseded (4),
cessationOfOperation (5),
certificateHold (6) }
4.2.2 Private Internet Extensions
This section defines one new extension for use in the Internet Public
Key Infrastructure. This extension may be used to direct
applications to identify an on-line validation service supporting the
issuing CA. As the information may be available in multiple forms,
each extension is a sequence of IA5String values, each of which
represents a URI. The URI implicitly specifies the location and
format of the information and the method for obtaining the
information.
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RFC 2459 Internet X.509 Public Key Infrastructure January 1999
An object identifier is defined for the private extension. The
object identifier associated with the private extension is defined
under the arc id-pe within the id-pkix name space. Any future
extensions defined for the Internet PKI will also be defined under
the arc id-pe.
id-pkix OBJECT IDENTIFIER ::=
{ iso(1) identified-organization(3) dod(6) internet(1)
security(5) mechanisms(5) pkix(7) }
id-pe OBJECT IDENTIFIER ::= { id-pkix 1 }
4.2.2.1 Authority Information Access
The authority information access extension indicates how to access CA
information and services for the issuer of the certificate in which
the extension appears. Information and services may include on-line
validation services and CA policy data. (The location of CRLs is not
specified in this extension; that information is provided by the
cRLDistributionPoints extension.) This extension may be included in
subject or CA certificates, and it MUST be non-critical.
id-pe-authorityInfoAccess OBJECT IDENTIFIER ::= { id-pe 1 }
AuthorityInfoAccessSyntax ::=
SEQUENCE SIZE (1..MAX) OF AccessDescription
AccessDescription ::= SEQUENCE {
accessMethod OBJECT IDENTIFIER,
accessLocation GeneralName }
id-ad OBJECT IDENTIFIER ::= { id-pkix 48 }
id-ad-caIssuers OBJECT IDENTIFIER ::= { id-ad 2 }
Each entry in the sequence AuthorityInfoAccessSyntax describes the
format and location of additional information about the CA who issued
the certificate in which this extension appears. The type and format
of the information is specified by the accessMethod field; the
accessLocation field specifies the location of the information. The
retrieval mechanism may be implied by the accessMethod or specified
by accessLocation.
This profile defines one OID for accessMethod. The id-ad-caIssuers
OID is used when the additional information lists CAs that have
issued certificates superior to the CA that issued the certificate
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RFC 2459 Internet X.509 Public Key Infrastructure January 1999
containing this extension. The referenced CA Issuers description is
intended to aid certificate users in the selection of a certification
path that terminates at a point trusted by the certificate user.
When id-ad-caIssuers appears as accessInfoType, the accessLocation
field describes the referenced description server and the access
protocol to obtain the referenced description. The accessLocation
field is defined as a GeneralName, which can take several forms.
Where the information is available via http, ftp, or ldap,
accessLocation MUST be a uniformResourceIdentifier. Where the
information is available via the directory access protocol (dap),
accessLocation MUST be a directoryName. When the information is
available via electronic mail, accessLocation MUST be an rfc822Name.
The semantics of other name forms of accessLocation (when
accessMethod is id-ad-caIssuers) are not defined by this
specification.
Additional access descriptors may be defined in other PKIX
specifications.
5 CRL and CRL Extensions Profile
As described above, one goal of this X.509 v2 CRL profile is to
foster the creation of an interoperable and reusable Internet PKI.
To achieve this goal, guidelines for the use of extensions are
specified, and some assumptions are made about the nature of
information included in the CRL.
CRLs may be used in a wide range of applications and environments
covering a broad spectrum of interoperability goals and an even
broader spectrum of operational and assurance requirements. This
profile establishes a common baseline for generic applications
requiring broad interoperability. The profile defines a baseline set
of information that can be expected in every CRL. Also, the profile
defines common locations within the CRL for frequently used
attributes as well as common representations for these attributes.
This profile does not define any private Internet CRL extensions or
CRL entry extensions.
Environments with additional or special purpose requirements may
build on this profile or may replace it.
Conforming CAs are not required to issue CRLs if other revocation or
certificate status mechanisms are provided. Conforming CAs that
issue CRLs MUST issue version 2 CRLs, and CAs MUST include the date
by which the next CRL will be issued in the nextUpdate field (see
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RFC 2459 Internet X.509 Public Key Infrastructure January 1999
sec. 5.1.2.5), the CRL number extension (see sec. 5.2.3) and the
authority key identifier extension (see sec. 5.2.1). Conforming
applications are required to process version 1 and 2 CRLs.
5.1 CRL Fields
The X.509 v2 CRL syntax is as follows. For signature calculation,
the data that is to be signed is ASN.1 DER encoded. ASN.1 DER
encoding is a tag, length, value encoding system for each element.
CertificateList ::= SEQUENCE {
tbsCertList TBSCertList,
signatureAlgorithm AlgorithmIdentifier,
signatureValue BIT STRING }
TBSCertList ::= SEQUENCE {
version Version OPTIONAL,
-- if present, shall be v2
signature AlgorithmIdentifier,
issuer Name,
thisUpdate Time,
nextUpdate Time OPTIONAL,
revokedCertificates SEQUENCE OF SEQUENCE {
userCertificate CertificateSerialNumber,
revocationDate Time,
crlEntryExtensions Extensions OPTIONAL
-- if present, shall be v2
} OPTIONAL,
crlExtensions [0] EXPLICIT Extensions OPTIONAL
-- if present, shall be v2
}
-- Version, Time, CertificateSerialNumber, and Extensions
-- are all defined in the ASN.1 in section 4.1
-- AlgorithmIdentifier is defined in section 4.1.1.2
The following items describe the use of the X.509 v2 CRL in the
Internet PKI.
5.1.1 CertificateList Fields
The CertificateList is a SEQUENCE of three required fields. The
fields are described in detail in the following subsections.
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RFC 2459 Internet X.509 Public Key Infrastructure January 19995.1.1.1 tbsCertList
The first field in the sequence is the tbsCertList. This field is
itself a sequence containing the name of the issuer, issue date,
issue date of the next list, the list of revoked certificates, and
optional CRL extensions. Further, each entry on the revoked
certificate list is defined by a sequence of user certificate serial
number, revocation date, and optional CRL entry extensions.
5.1.1.2 signatureAlgorithm
The signatureAlgorithm field contains the algorithm identifier for
the algorithm used by the CA to sign the CertificateList. The field
is of type AlgorithmIdentifier, which is defined in section 4.1.1.2.
Section 7.2 lists the supported algorithms for this specification.
Conforming CAs MUST use the algorithm identifiers presented in
section 7.2 when signing with a supported signature algorithm.
This field MUST contain the same algorithm identifier as the
signature field in the sequence tbsCertList (see sec. 5.1.2.2).
5.1.1.3 signatureValue
The signatureValue field contains a digital signature computed upon
the ASN.1 DER encoded tbsCertList. The ASN.1 DER encoded tbsCertList
is used as the input to the signature function. This signature value
is then ASN.1 encoded as a BIT STRING and included in the CRL's
signatureValue field. The details of this process are specified for
each of the supported algorithms in section 7.2.
5.1.2 Certificate List "To Be Signed"
The certificate list to be signed, or TBSCertList, is a SEQUENCE of
required and optional fields. The required fields identify the CRL
issuer, the algorithm used to sign the CRL, the date and time the CRL
was issued, and the date and time by which the CA will issue the next
CRL.
Optional fields include lists of revoked certificates and CRL
extensions. The revoked certificate list is optional to support the
case where a CA has not revoked any unexpired certificates that it
has issued. The profile requires conforming CAs to use the CRL
extension cRLNumber in all CRLs issued.
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RFC 2459 Internet X.509 Public Key Infrastructure January 19995.1.2.1 Version
This optional field describes the version of the encoded CRL. When
extensions are used, as required by this profile, this field MUST be
present and MUST specify version 2 (the integer value is 1).
5.1.2.2 Signature
This field contains the algorithm identifier for the algorithm used
to sign the CRL. Section 7.2 lists OIDs for the most popular
signature algorithms used in the Internet PKI.
This field MUST contain the same algorithm identifier as the
signatureAlgorithm field in the sequence CertificateList (see section5.1.1.2).
5.1.2.3 Issuer Name
The issuer name identifies the entity who has signed and issued the
CRL. The issuer identity is carried in the issuer name field.
Alternative name forms may also appear in the issuerAltName extension
(see sec. 5.2.2). The issuer name field MUST contain an X.500
distinguished name (DN). The issuer name field is defined as the
X.501 type Name, and MUST follow the encoding rules for the issuer
name field in the certificate (see sec. 4.1.2.4).
5.1.2.4 This Update
This field indicates the issue date of this CRL. ThisUpdate may be
encoded as UTCTime or GeneralizedTime.
CAs conforming to this profile that issue CRLs MUST encode thisUpdate
as UTCTime for dates through the year 2049. CAs conforming to this
profile that issue CRLs MUST encode thisUpdate as GeneralizedTime for
dates in the year 2050 or later.
Where encoded as UTCTime, thisUpdate MUST be specified and
interpreted as defined in section 4.1.2.5.1. Where encoded as
GeneralizedTime, thisUpdate MUST be specified and interpreted as
defined in section 4.1.2.5.2.
5.1.2.5 Next Update
This field indicates the date by which the next CRL will be issued.
The next CRL could be issued before the indicated date, but it will
not be issued any later than the indicated date. CAs SHOULD issue
CRLs with a nextUpdate time equal to or later than all previous CRLs.
nextUpdate may be encoded as UTCTime or GeneralizedTime.
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RFC 2459 Internet X.509 Public Key Infrastructure January 1999
This profile requires inclusion of nextUpdate in all CRLs issued by
conforming CAs. Note that the ASN.1 syntax of TBSCertList describes
this field as OPTIONAL, which is consistent with the ASN.1 structure
defined in [X.509]. The behavior of clients processing CRLs which
omit nextUpdate is not specified by this profile.
CAs conforming to this profile that issue CRLs MUST encode nextUpdate
as UTCTime for dates through the year 2049. CAs conforming to this
profile that issue CRLs MUST encode nextUpdate as GeneralizedTime for
dates in the year 2050 or later.
Where encoded as UTCTime, nextUpdate MUST be specified and
interpreted as defined in section 4.1.2.5.1. Where encoded as
GeneralizedTime, nextUpdate MUST be specified and interpreted as
defined in section 4.1.2.5.2.
5.1.2.6 Revoked Certificates
Revoked certificates are listed. The revoked certificates are named
by their serial numbers. Certificates revoked by the CA are uniquely
identified by the certificate serial number. The date on which the
revocation occurred is specified. The time for revocationDate MUST
be expressed as described in section 5.1.2.4. Additional information
may be supplied in CRL entry extensions; CRL entry extensions are
discussed in section 5.3.
5.1.2.7 Extensions
This field may only appear if the version is 2 (see sec. 5.1.2.1).
If present, this field is a SEQUENCE of one or more CRL extensions.
CRL extensions are discussed in section 5.2.
5.2 CRL Extensions
The extensions defined by ANSI X9 and ISO/IEC/ITU for X.509 v2 CRLs
[X.509] [X9.55] provide methods for associating additional attributes
with CRLs. The X.509 v2 CRL format also allows communities to define
private extensions to carry information unique to those communities.
Each extension in a CRL may be designated as critical or non-
critical. A CRL validation MUST fail if it encounters a critical
extension which it does not know how to process. However, an
unrecognized non-critical extension may be ignored. The following
subsections present those extensions used within Internet CRLs.
Communities may elect to include extensions in CRLs which are not
defined in this specification. However, caution should be exercised
in adopting any critical extensions in CRLs which might be used in a
general context.
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RFC 2459 Internet X.509 Public Key Infrastructure January 1999
Conforming CAs that issue CRLs are required to include the authority
key identifier (see sec. 5.2.1) and the CRL number (see sec. 5.2.3)
extensions in all CRLs issued.
5.2.1 Authority Key Identifier
The authority key identifier extension provides a means of
identifying the public key corresponding to the private key used to
sign a CRL. The identification can be based on either the key
identifier (the subject key identifier in the CRL signer's
certificate) or on the issuer name and serial number. This extension
is especially useful where an issuer has more than one signing key,
either due to multiple concurrent key pairs or due to changeover.
Conforming CAs MUST use the key identifier method, and MUST include
this extension in all CRLs issued.
The syntax for this CRL extension is defined in section 4.2.1.1.
5.2.2 Issuer Alternative Name
The issuer alternative names extension allows additional identities
to be associated with the issuer of the CRL. Defined options include
an rfc822 name (electronic mail address), a DNS name, an IP address,
and a URI. Multiple instances of a name and multiple name forms may
be included. Whenever such identities are used, the issuer
alternative name extension MUST be used.
The issuerAltName extension SHOULD NOT be marked critical.
The OID and syntax for this CRL extension are defined in section4.2.1.8.
5.2.3 CRL Number
The CRL number is a non-critical CRL extension which conveys a
monotonically increasing sequence number for each CRL issued by a CA.
This extension allows users to easily determine when a particular CRL
supersedes another CRL. CAs conforming to this profile MUST include
this extension in all CRLs.
id-ce-cRLNumber OBJECT IDENTIFIER ::= { id-ce 20 }
cRLNumber ::= INTEGER (0..MAX)
Housley, et. al. Standards Track [Page 47]

RFC 2459 Internet X.509 Public Key Infrastructure January 19995.2.4 Delta CRL Indicator
The delta CRL indicator is a critical CRL extension that identifies a
delta-CRL. The use of delta-CRLs can significantly improve
processing time for applications which store revocation information
in a format other than the CRL structure. This allows changes to be
added to the local database while ignoring unchanged information that
is already in the local database.
When a delta-CRL is issued, the CAs MUST also issue a complete CRL.
The value of BaseCRLNumber identifies the CRL number of the base CRL
that was used as the starting point in the generation of this delta-
CRL. The delta-CRL contains the changes between the base CRL and the
current CRL issued along with the delta-CRL. It is the decision of a
CA as to whether to provide delta-CRLs. Again, a delta-CRL MUST NOT
be issued without a corresponding complete CRL. The value of
CRLNumber for both the delta-CRL and the corresponding complete CRL
MUST be identical.
A CRL user constructing a locally held CRL from delta-CRLs MUST
consider the constructed CRL incomplete and unusable if the CRLNumber
of the received delta-CRL is more than one greater than the CRLnumber
of the delta-CRL last processed.
id-ce-deltaCRLIndicator OBJECT IDENTIFIER ::= { id-ce 27 }
deltaCRLIndicator ::= BaseCRLNumber
BaseCRLNumber ::= CRLNumber
5.2.5 Issuing Distribution Point
The issuing distribution point is a critical CRL extension that
identifies the CRL distribution point for a particular CRL, and it
indicates whether the CRL covers revocation for end entity
certificates only, CA certificates only, or a limitied set of reason
codes. Although the extension is critical, conforming
implementations are not required to support this extension.
The CRL is signed using the CA's private key. CRL Distribution
Points do not have their own key pairs. If the CRL is stored in the
X.500 Directory, it is stored in the Directory entry corresponding to
the CRL distribution point, which may be different than the Directory
entry of the CA.
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RFC 2459 Internet X.509 Public Key Infrastructure January 1999
The reason codes associated with a distribution point shall be
specified in onlySomeReasons. If onlySomeReasons does not appear, the
distribution point shall contain revocations for all reason codes.
CAs may use CRL distribution points to partition the CRL on the basis
of compromise and routine revocation. In this case, the revocations
with reason code keyCompromise (1) and cACompromise (2) appear in one
distribution point, and the revocations with other reason codes
appear in another distribution point.
Where the issuingDistributionPoint extension contains a URL, the
following semantics MUST be assumed: the object is a pointer to the
most current CRL issued by this CA. The URI schemes ftp, http,
mailto [RFC1738] and ldap [RFC1778] are defined for this purpose.
The URI MUST be an absolute, not relative, pathname and MUST specify
the host.
id-ce-issuingDistributionPoint OBJECT IDENTIFIER ::= { id-ce 28 }
issuingDistributionPoint ::= SEQUENCE {
distributionPoint [0] DistributionPointName OPTIONAL,
onlyContainsUserCerts [1] BOOLEAN DEFAULT FALSE,
onlyContainsCACerts [2] BOOLEAN DEFAULT FALSE,
onlySomeReasons [3] ReasonFlags OPTIONAL,
indirectCRL [4] BOOLEAN DEFAULT FALSE }
5.3 CRL Entry Extensions
The CRL entry extensions already defined by ANSI X9 and ISO/IEC/ITU
for X.509 v2 CRLs provide methods for associating additional
attributes with CRL entries [X.509] [X9.55]. The X.509 v2 CRL format
also allows communities to define private CRL entry extensions to
carry information unique to those communities. Each extension in a
CRL entry may be designated as critical or non-critical. A CRL
validation MUST fail if it encounters a critical CRL entry extension
which it does not know how to process. However, an unrecognized
non-critical CRL entry extension may be ignored. The following
subsections present recommended extensions used within Internet CRL
entries and standard locations for information. Communities may
elect to use additional CRL entry extensions; however, caution should
be exercised in adopting any critical extensions in CRL entries which
might be used in a general context.
All CRL entry extensions used in this specification are non-critical.
Support for these extensions is optional for conforming CAs and
applications. However, CAs that issue CRLs SHOULD include reason
codes (see sec. 5.3.1) and invalidity dates (see sec. 5.3.3) whenever
this information is available.
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RFC 2459 Internet X.509 Public Key Infrastructure January 1999
holdinstruction-reject MUST reject the certificate. The hold
instruction id-holdinstruction-none is semantically equivalent to the
absence of a holdInstructionCode, and its use is strongly deprecated
for the Internet PKI.
5.3.3 Invalidity Date
The invalidity date is a non-critical CRL entry extension that
provides the date on which it is known or suspected that the private
key was compromised or that the certificate otherwise became invalid.
This date may be earlier than the revocation date in the CRL entry,
which is the date at which the CA processed the revocation. When a
revocation is first posted by a CA in a CRL, the invalidity date may
precede the date of issue of earlier CRLs, but the revocation date
SHOULD NOT precede the date of issue of earlier CRLs. Whenever this
information is available, CAs are strongly encouraged to share it
with CRL users.
The GeneralizedTime values included in this field MUST be expressed
in Greenwich Mean Time (Zulu), and MUST be specified and interpreted
as defined in section 4.1.2.5.2.
id-ce-invalidityDate OBJECT IDENTIFIER ::= { id-ce 24 }
invalidityDate ::= GeneralizedTime
5.3.4 Certificate Issuer
This CRL entry extension identifies the certificate issuer associated
with an entry in an indirect CRL, i.e. a CRL that has the indirectCRL
indicator set in its issuing distribution point extension. If this
extension is not present on the first entry in an indirect CRL, the
certificate issuer defaults to the CRL issuer. On subsequent entries
in an indirect CRL, if this extension is not present, the certificate
issuer for the entry is the same as that for the preceding entry.
This field is defined as follows:
id-ce-certificateIssuer OBJECT IDENTIFIER ::= { id-ce 29 }
certificateIssuer ::= GeneralNames
If used by conforming CAs that issue CRLs, this extension is always
critical. If an implementation ignored this extension it could not
correctly attribute CRL entries to certificates. This specification
RECOMMENDS that implementations recognize this extension.
Housley, et. al. Standards Track [Page 51]

RFC 2459 Internet X.509 Public Key Infrastructure January 19996 Certification Path Validation
Certification path validation procedures for the Internet PKI are
based on section 12.4.3 of [X.509]. Certification path processing
verifies the binding between the subject distinguished name and/or
subject alternative name and subject public key. The binding is
limited by constraints which are specified in the certificates which
comprise the path. The basic constraints and policy constraints
extensions allow the certification path processing logic to automate
the decision making process.
This section describes an algorithm for validating certification
paths. Conforming implementations of this specification are not
required to implement this algorithm, but MUST be functionally
equivalent to the external behavior resulting from this procedure.
Any algorithm may be used by a particular implementation so long as
it derives the correct result.
In section 6.1, the text describes basic path validation. This text
assumes that all valid paths begin with certificates issued by a
single "most-trusted CA". The algorithm requires the public key of
the CA, the CA's name, the validity period of the public key, and any
constraints upon the set of paths which may be validated using this
key.
The "most-trusted CA" is a matter of policy: it could be a root CA in
a hierarchical PKI; the CA that issued the verifier's own
certificate(s); or any other CA in a network PKI. The path
validation procedure is the same regardless of the choice of "most-
trusted CA."
section 6.2 describes extensions to the basic path validation
algorithm. Two specific cases are discussed: the case where paths may
begin with one of several trusted CAs; and where compatibility with
the PEM architecture is required.
6.1 Basic Path Validation
The text assumes that the trusted public key (and related
information) is contained in a "self-signed" certificate. This
simplifies the description of the path processing procedure. Note
that the signature on the self-signed certificate does not provide
any security services. The trusted public key (and related
information) may be obtained in other formats; the information is
trusted because of other procedures used to obtain and protect it.
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RFC 2459 Internet X.509 Public Key Infrastructure January 1999
The goal of path validation is to verify the binding between a
subject distinguished name or subject alternative name and subject
public key, as represented in the "end entity" certificate, based on
the public key of the "most-trusted CA". This requires obtaining a
sequence of certificates that support that binding. The procedures
performed to obtain this sequence is outside the scope of this
section.
The following text also assumes that certificates do not use subject
or unique identifier fields or private critical extensions, as
recommended within this profile. However, if these components appear
in certificates, they MUST be processed. Finally, policy qualifiers
are also neglected for the sake of clarity.
A certification path is a sequence of n certificates where:
* for all x in {1,(n-1)}, the subject of certificate x is the
issuer of certificate x+1.
* certificate x=1 is the the self-signed certificate, and
* certificate x=n is the end entity certificate.
This section assumes the following inputs are provided to the path
processing logic:
(a) a certification path of length n;
(b) a set of initial policy identifiers (each comprising a
sequence of policy element identifiers), which identifies one or
more certificate policies, any one of which would be acceptable
for the purposes of certification path processing, or the special
value "any-policy";
(c) the current date/time (if not available internally to the
certification path processing module); and
(d) the time, T, for which the validity of the path should be
determined. (This may be the current date/time, or some point in
the past.)
From the inputs, the procedure intializes five state variables:
(a) acceptable policy set: A set of certificate policy
identifiers comprising the policy or policies recognized by the
public key user together with policies deemed equivalent through
policy mapping. The initial value of the acceptable policy set is
the special value "any-policy".
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RFC 2459 Internet X.509 Public Key Infrastructure January 1999
(b) constrained subtrees: A set of root names defining a set of
subtrees within which all subject names in subsequent certificates
in the certification path shall fall. The initial value is
"unbounded".
(c) excluded subtrees: A set of root names defining a set of
subtrees within which no subject name in subsequent certificates
in the certification path may fall. The initial value is "empty".
(d) explicit policy: an integer which indicates if an explicit
policy identifier is required. The integer indicates the first
certificate in the path where this requirement is imposed. Once
set, this variable may be decreased, but may not be increased.
(That is, if a certificate in the path requires explicit policy
identifiers, a later certificate can not remove this requirement.)
The initial value is n+1.
(e) policy mapping: an integer which indicates if policy mapping
is permitted. The integer indicates the last certificate on which
policy mapping may be applied. Once set, this variable may be
decreased, but may not be increased. (That is, if a certificate in
the path specifies policy mapping is not permitted, it can not be
overriden by a later certificate.) The initial value is n+1.
The actions performed by the path processing software for each
certificate i=1 through n are described below. The self-signed
certificate is certificate i=1, the end entity certificate is i=n.
The processing is performed sequentially, so that processing
certificate i affects the state variables for processing certificate
(i+1). Note that actions (h) through (m) are not applied to the end
entity certificate (certificate n).
The path processing actions to be performed are:
(a) Verify the basic certificate information, including:
(1) the certificate was signed using the subject public key
from certificate i-1 (in the special case i=1, this step may be
omitted; if not, use the subject public key from the same
certificate),
(2) the certificate validity period includes time T,
(3) the certificate had not been revoked at time T and is not
currently on hold status that commenced before time T, (this
may be determined by obtaining the appropriate CRL or status
information, or by out-of-band mechanisms), and
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RFC 2459 Internet X.509 Public Key Infrastructure January 1999
(4) the subject and issuer names chain correctly (that is, the
issuer of this certificate was the subject of the previous
certificate.)
(b) Verify that the subject name and subjectAltName extension
(critical or noncritical) is consistent with the constrained
subtrees state variables.
(c) Verify that the subject name and subjectAltName extension
(critical or noncritical) is consistent with the excluded subtrees
state variables.
(d) Verify that policy information is consistent with the initial
policy set:
(1) if the explicit policy state variable is less than or equal
to i, a policy identifier in the certificate shall be in the
initial policy set; and
(2) if the policy mapping variable is less than or equal to i,
the policy identifier may not be mapped.
(e) Verify that policy information is consistent with the
acceptable policy set:
(1) if the certificate policies extension is marked critical,
the intersection of the policies extension and the acceptable
policy set shall be non-null;
(2) the acceptable policy set is assigned the resulting
intersection as its new value.
(g) Verify that the intersection of the acceptable policy set and
the initial policy set is non-null.
(h) Recognize and process any other critical extension present in
the certificate.
(i) Verify that the certificate is a CA certificate (as specified
in a basicConstraints extension or as verified out-of-band).
(j) If permittedSubtrees is present in the certificate, set the
constrained subtrees state variable to the intersection of its
previous value and the value indicated in the extension field.
(k) If excludedSubtrees is present in the certificate, set the
excluded subtrees state variable to the union of its previous
value and the value indicated in the extension field.
Housley, et. al. Standards Track [Page 55]

RFC 2459 Internet X.509 Public Key Infrastructure January 1999
(l) If a policy constraints extension is included in the
certificate, modify the explicit policy and policy mapping state
variables as follows:
(1) If requireExplicitPolicy is present and has value r, the
explicit policy state variable is set to the minimum of its
current value and the sum of r and i (the current certificate
in the sequence).
(2) If inhibitPolicyMapping is present and has value q, the
policy mapping state variable is set to the minimum of its
current value and the sum of q and i (the current certificate
in the sequence).
(m) If a key usage extension is marked critical, ensure the
keyCertSign bit is set.
If any one of the above checks fail, the procedure terminates,
returning a failure indication and an appropriate reason. If none of
the above checks fail on the end-entity certificate, the procedure
terminates, returning a success indication together with the set of
all policy qualifier values encountered in the set of certificates.
6.2 Extending Path Validation
The path validation algorithm presented in 6.1 is based on several
simplifying assumptions (e.g., a single trusted CA that starts all
valid paths). This algorithm may be extended for cases where the
assumptions do not hold.
This procedure may be extended for multiple trusted CAs by providing
a set of self-signed certificates to the validation module. In this
case, a valid path could begin with any one of the self-signed
certificates. Limitations in the trust paths for any particular key
may be incorporated into the self-signed certificate's extensions. In
this way, the self-signed certificates permit the path validation
module to automatically incorporate local security policy and
requirements.
It is also possible to specify an extended version of the above
certification path processing procedure which results in default
behavior identical to the rules of PEM [RFC 1422]. In this extended
version, additional inputs to the procedure are a list of one or more
Policy Certification Authorities (PCAs) names and an indicator of the
position in the certification path where the PCA is expected. At the
nominated PCA position, the CA name is compared against this list.
If a recognized PCA name is found, then a constraint of
SubordinateToCA is implicitly assumed for the remainder of the
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RFC 2459 Internet X.509 Public Key Infrastructure January 1999
certification path and processing continues. If no valid PCA name is
found, and if the certification path cannot be validated on the basis
of identified policies, then the certification path is considered
invalid.
7 Algorithm Support
This section describes cryptographic algorithms which may be used
with this profile. The section describes one-way hash functions and
digital signature algorithms which may be used to sign certificates
and CRLs, and identifies OIDs for public keys contained in a
certificate.
Conforming CAs and applications are not required to support the
algorithms or algorithm identifiers described in this section.
However, conforming CAs and applications that use the algorithms
identified here MUST support them as specified.
7.1 One-way Hash Functions
This section identifies one-way hash functions for use in the
Internet PKI. One-way hash functions are also called message digest
algorithms. SHA-1 is the preferred one-way hash function for the
Internet PKI. However, PEM uses MD2 for certificates [RFC 1422] [RFC
1423] and MD5 is used in other legacy applications. For this reason,
MD2 and MD5 are included in this profile.
7.1.1 MD2 One-way Hash Function
MD2 was developed by Ron Rivest for RSA Data Security. RSA Data
Security has not placed the MD2 algorithm in the public domain.
Rather, RSA Data Security has granted license to use MD2 for non-
commercial Internet Privacy-Enhanced Mail. For this reason, MD2 may
continue to be used with PEM certificates, but SHA-1 is preferred.
MD2 produces a 128-bit "hash" of the input. MD2 is fully described
in RFC 1319 [RFC 1319].
At the Selected Areas in Cryptography '95 conference in May 1995,
Rogier and Chauvaud presented an attack on MD2 that can nearly find
collisions [RC95]. Collisions occur when one can find two different
messages that generate the same message digest. A checksum operation
in MD2 is the only remaining obstacle to the success of the attack.
For this reason, the use of MD2 for new applications is discouraged.
It is still reasonable to use MD2 to verify existing signatures, as
the ability to find collisions in MD2 does not enable an attacker to
find new messages having a previously computed hash value.
Housley, et. al. Standards Track [Page 57]

RFC 2459 Internet X.509 Public Key Infrastructure January 19997.1.2 MD5 One-way Hash Function
MD5 was developed by Ron Rivest for RSA Data Security. RSA Data
Security has placed the MD5 algorithm in the public domain. MD5
produces a 128-bit "hash" of the input. MD5 is fully described in
RFC 1321 [RFC 1321].
Den Boer and Bosselaers [DB94] have found pseudo-collisions for MD5,
but there are no other known cryptanalytic results. The use of MD5
for new applications is discouraged. It is still reasonable to use
MD5 to verify existing signatures.
7.1.3 SHA-1 One-way Hash Function
SHA-1 was developed by the U.S. Government. SHA-1 produces a 160-bit
"hash" of the input. SHA-1 is fully described in FIPS 180-1 [FIPS
180-1].
SHA-1 is the one-way hash function of choice for use with both the
RSA and DSA signature algorithms (see sec. 7.2).
7.2 Signature Algorithms
Certificates and CRLs described by this standard may be signed with
any public key signature algorithm. The certificate or CRL indicates
the algorithm through an algorithm identifier which appears in the
signatureAlgorithm field in a Certificate or CertificateList. This
algorithm identifier is an OID and has optionally associated
parameters. This section identifies algorithm identifiers and
parameters that shall be used in the signatureAlgorithm field in a
Certificate or CertificateList.
RSA and DSA are the most popular signature algorithms used in the
Internet. Signature algorithms are always used in conjunction with a
one-way hash function identified in section 7.1.
The signature algorithm and one-way hash function used to sign a
certificate or CRL is indicated by use of an algorithm identifier.
An algorithm identifier is an OID, and may include associated
parameters. This section identifies OIDS for RSA and DSA. The
contents of the parameters component for each algorithm vary; details
are provided for each algorithm.
The data to be signed (e.g., the one-way hash function output value)
is formatted for the signature algorithm to be used. Then, a private
key operation (e.g., RSA encryption) is performed to generate the
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RFC 2459 Internet X.509 Public Key Infrastructure January 1999
signature value. This signature value is then ASN.1 encoded as a BIT
STRING and included in the Certificate or CertificateList in the
signature field.
7.2.1 RSA Signature Algorithm
A patent statement regarding the RSA algorithm can be found at the
end of this profile.
The RSA algorithm is named for its inventors: Rivest, Shamir, and
Adleman. This profile includes three signature algorithms based on
the RSA asymmetric encryption algorithm. The signature algorithms
combine RSA with either the MD2, MD5, or the SHA-1 one-way hash
functions.
The signature algorithm with MD2 and the RSA encryption algorithm is
defined in PKCS #1 [RFC 2313]. As defined in RFC 2313, the ASN.1 OID
used to identify this signature algorithm is:
md2WithRSAEncryption OBJECT IDENTIFIER ::= {
iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1)
pkcs-1(1) 2 }
The signature algorithm with MD5 and the RSA encryption algorithm is
defined in PKCS #1 [RFC 2313]. As defined in RFC 2313, the ASN.1 OID
used to identify this signature algorithm is:
md5WithRSAEncryption OBJECT IDENTIFIER ::= {
iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1)
pkcs-1(1) 4 }
The signature algorithm with SHA-1 and the RSA encryption algorithm
is implemented using the padding and encoding conventions described
in PKCS #1 [RFC 2313]. The message digest is computed using the SHA-1
hash algorithm. The ASN.1 object identifier used to identify this
signature algorithm is:
sha-1WithRSAEncryption OBJECT IDENTIFIER ::= {
iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1)
pkcs-1(1) 5 }
When any of these three OIDs appears within the ASN.1 type
AlgorithmIdentifier, the parameters component of that type shall be
the ASN.1 type NULL.
The RSA signature generation process and the encoding of the result
is described in detail in RFC 2313.
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RFC 2459 Internet X.509 Public Key Infrastructure January 19997.2.2 DSA Signature Algorithm
A patent statement regarding the DSA can be found at the end of this
profile.
The Digital Signature Algorithm (DSA) is also called the Digital
Signature Standard (DSS). DSA was developed by the U.S. Government,
and DSA is used in conjunction with the the SHA-1 one-way hash
function. DSA is fully described in FIPS 186 [FIPS 186]. The ASN.1
OIDs used to identify this signature algorithm are:
id-dsa-with-sha1 ID ::= {
iso(1) member-body(2) us(840) x9-57 (10040)
x9cm(4) 3 }
Where the id-dsa-with-sha1 algorithm identifier appears as the
algorithm field in an AlgorithmIdentifier, the encoding shall omit
the parameters field. That is, the AlgorithmIdentifier shall be a
SEQUENCE of one component - the OBJECT IDENTIFIER id-dsa-with-sha1.
The DSA parameters in the subjectPublicKeyInfo field of the
certificate of the issuer shall apply to the verification of the
signature.
When signing, the DSA algorithm generates two values. These values
are commonly referred to as r and s. To easily transfer these two
values as one signature, they shall be ASN.1 encoded using the
following ASN.1 structure:
Dss-Sig-Value ::= SEQUENCE {
r INTEGER,
s INTEGER }
7.3 Subject Public Key Algorithms
Certificates described by this profile may convey a public key for
any public key algorithm. The certificate indicates the algorithm
through an algorithm identifier. This algorithm identifier is an OID
and optionally associated parameters.
This section identifies preferred OIDs and parameters for the RSA,
DSA, and Diffie-Hellman algorithms. Conforming CAs shall use the
identified OIDs when issuing certificates containing public keys for
these algorithms. Conforming applications supporting any of these
algorithms shall, at a minimum, recognize the OID identified in this
section.
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RFC 2459 Internet X.509 Public Key Infrastructure January 19997.3.1 RSA Keys
The OID rsaEncryption identifies RSA public keys.
pkcs-1 OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840)
rsadsi(113549) pkcs(1) 1 }
rsaEncryption OBJECT IDENTIFIER ::= { pkcs-1 1}
The rsaEncryption OID is intended to be used in the algorithm field
of a value of type AlgorithmIdentifier. The parameters field shall
have ASN.1 type NULL for this algorithm identifier.
The RSA public key shall be encoded using the ASN.1 type
RSAPublicKey:
RSAPublicKey ::= SEQUENCE {
modulus INTEGER, -- n
publicExponent INTEGER -- e -- }
where modulus is the modulus n, and publicExponent is the public
exponent e. The DER encoded RSAPublicKey is the value of the BIT
STRING subjectPublicKey.
This OID is used in public key certificates for both RSA signature
keys and RSA encryption keys. The intended application for the key
may be indicated in the key usage field (see sec. 4.2.1.3). The use
of a single key for both signature and encryption purposes is not
recommended, but is not forbidden.
If the keyUsage extension is present in an end entity certificate
which conveys an RSA public key, any combination of the following
values may be present: digitalSignature; nonRepudiation;
keyEncipherment; and dataEncipherment. If the keyUsage extension is
present in a CA certificate which conveys an RSA public key, any
combination of the following values may be present:
digitalSignature; nonRepudiation; keyEncipherment; dataEncipherment;
keyCertSign; and cRLSign. However, this specification RECOMMENDS
that if keyCertSign or cRLSign is present, both keyEncipherment and
dataEncipherment should not be present.
7.3.2 Diffie-Hellman Key Exchange Key
The Diffie-Hellman OID supported by this profile is defined by ANSI
X9.42 [X9.42].
dhpublicnumber OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) ansi-x942(10046) number-type(2) 1 }
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RFC 2459 Internet X.509 Public Key Infrastructure January 1999
The dhpublicnumber OID is intended to be used in the algorithm field
of a value of type AlgorithmIdentifier. The parameters field of that
type, which has the algorithm-specific syntax ANY DEFINED BY
algorithm, have the ASN.1 type DomainParameters for this algorithm.
DomainParameters ::= SEQUENCE {
p INTEGER, -- odd prime, p=jq +1
g INTEGER, -- generator, g
q INTEGER, -- factor of p-1
j INTEGER OPTIONAL, -- subgroup factor
validationParms ValidationParms OPTIONAL }
ValidationParms ::= SEQUENCE {
seed BIT STRING,
pgenCounter INTEGER }
The fields of type DomainParameters have the following meanings:
p identifies the prime p defining the Galois field;
g specifies the generator of the multiplicative subgroup of order
g;
q specifies the prime factor of p-1;
j optionally specifies the value that satisfies the equation
p=jq+1 to support the optional verification of group parameters;
seed optionally specifies the bit string parameter used as the
seed for the system parameter generation process; and
pgenCounter optionally specifies the integer value output as part
of the of the system parameter prime generation process.
If either of the parameter generation components (pgencounter or
seed) is provided, the other shall be present as well.
The Diffie-Hellman public key shall be ASN.1 encoded as an INTEGER;
this encoding shall be used as the contents (i.e., the value) of the
subjectPublicKey component (a BIT STRING) of the subjectPublicKeyInfo
data element.
DHPublicKey ::= INTEGER -- public key, y = g^x mod p
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RFC 2459 Internet X.509 Public Key Infrastructure January 1999
If the keyUsage extension is present in a certificate which conveys a
DH public key, the following values may be present: keyAgreement;
encipherOnly; and decipherOnly. At most one of encipherOnly and
decipherOnly shall be asserted in keyUsage extension.
7.3.3 DSA Signature Keys
The Digital Signature Algorithm (DSA) is also known as the Digital
Signature Standard (DSS). The DSA OID supported by this profile is
id-dsa ID ::= { iso(1) member-body(2) us(840) x9-57(10040)
x9cm(4) 1 }
The id-dsa algorithm syntax includes optional parameters. These
parameters are commonly referred to as p, q, and g. When omitted,
the parameters component shall be omitted entirely. That is, the
AlgorithmIdentifier shall be a SEQUENCE of one component - the OBJECT
IDENTIFIER id-dsa.
If the DSA algorithm parameters are present in the
subjectPublicKeyInfo AlgorithmIdentifier, the parameters are included
using the following ASN.1 structure:
Dss-Parms ::= SEQUENCE {
p INTEGER,
q INTEGER,
g INTEGER }
If the DSA algorithm parameters are absent from the
subjectPublicKeyInfo AlgorithmIdentifier and the CA signed the
subject certificate using DSA, then the certificate issuer's DSA
parameters apply to the subject's DSA key. If the DSA algorithm
parameters are absent from the subjectPublicKeyInfo
AlgorithmIdentifier and the CA signed the subject certificate using a
signature algorithm other than DSA, then the subject's DSA parameters
are distributed by other means. If the subjectPublicKeyInfo
AlgorithmIdentifier field omits the parameters component and the CA
signed the subject with a signature algorithm other than DSA, then
clients shall reject the certificate.
When signing, DSA algorithm generates two values. These values are
commonly referred to as r and s. To easily transfer these two values
as one signature, they are ASN.1 encoded using the following ASN.1
structure:
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RFC 2459 Internet X.509 Public Key Infrastructure January 1999
[X.501] ITU-T Recommendation X.501: Information Technology -
Open Systems Interconnection - The Directory: Models,
1993.
[X.509] ITU-T Recommendation X.509 (1997 E): Information
Technology - Open Systems Interconnection - The
Directory: Authentication Framework, June 1997.
[X.520] ITU-T Recommendation X.520: Information Technology -
Open Systems Interconnection - The Directory: Selected
Attribute Types, 1993.
[X9.42] ANSI X9.42-199x, Public Key Cryptography for The
Financial Services Industry: Agreement of Symmetric
Algorithm Keys Using Diffie-Hellman (Working Draft),
December 1997.
[X9.55] ANSI X9.55-1995, Public Key Cryptography For The
Financial Services Industry: Extensions To Public Key
Certificates And Certificate Revocation Lists, 8
December, 1995.
[X9.57] ANSI X9.57-199x, Public Key Cryptography For The
Financial Services Industry: Certificate Management
(Working Draft), 21 June, 1996.
9 Intellectual Property Rights
The IETF has been notified of intellectual property rights claimed in
regard to some or all of the specification contained in this
document. For more information consult the online list of claimed
rights.
The IETF takes no position regarding the validity or scope of any
intellectual property or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; neither does it represent that it
has made any effort to identify any such rights. Information on the
IETF's procedures with respect to rights in standards-track and
standards-related documentation can be found in BCP-11. Copies of
claims of rights made available for publication and any assurances of
licenses to be made available, or the result of an attempt made to
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RFC 2459 Internet X.509 Public Key Infrastructure January 1999
obtain a general license or permission for the use of such
proprietary rights by implementors or users of this specification can
be obtained from the IETF Secretariat.
10 Security Considerations
The majority of this specification is devoted to the format and
content of certificates and CRLs. Since certificates and CRLs are
digitally signed, no additional integrity service is necessary.
Neither certificates nor CRLs need be kept secret, and unrestricted
and anonymous access to certificates and CRLs has no security
implications.
However, security factors outside the scope of this specification
will affect the assurance provided to certificate users. This
section highlights critical issues that should be considered by
implementors, administrators, and users.
The procedures performed by CAs and RAs to validate the binding of
the subject's identity of their public key greatly affect the
assurance that should be placed in the certificate. Relying parties
may wish to review the CA's certificate practice statement. This may
be particularly important when issuing certificates to other CAs.
The use of a single key pair for both signature and other purposes is
strongly discouraged. Use of separate key pairs for signature and key
management provides several benefits to the users. The ramifications
associated with loss or disclosure of a signature key are different
from loss or disclosure of a key management key. Using separate key
pairs permits a balanced and flexible response. Similarly, different
validity periods or key lengths for each key pair may be appropriate
in some application environments. Unfortunately, some legacy
applications (e.g., SSL) use a single key pair for signature and key
management.
The protection afforded private keys is a critical factor in
maintaining security. On a small scale, failure of users to protect
their private keys will permit an attacker to masquerade as them, or
decrypt their personal information. On a larger scale, compromise of
a CA's private signing key may have a catastrophic effect. If an
attacker obtains the private key unnoticed, the attacker may issue
bogus certificates and CRLs. Existence of bogus certificates and
CRLs will undermine confidence in the system. If the compromise is
detected, all certificates issued to the CA shall be revoked,
preventing services between its users and users of other CAs.
Rebuilding after such a compromise will be problematic, so CAs are
advised to implement a combination of strong technical measures
(e.g., tamper-resistant cryptographic modules) and appropriate
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RFC 2459 Internet X.509 Public Key Infrastructure January 1999
management procedures (e.g., separation of duties) to avoid such an
incident.
Loss of a CA's private signing key may also be problematic. The CA
would not be able to produce CRLs or perform normal key rollover.
CAs are advised to maintain secure backup for signing keys. The
security of the key backup procedures is a critical factor in
avoiding key compromise.
The availability and freshness of revocation information will affect
the degree of assurance that should be placed in a certificate.
While certificates expire naturally, events may occur during its
natural lifetime which negate the binding between the subject and
public key. If revocation information is untimely or unavailable,
the assurance associated with the binding is clearly reduced.
Similarly, implementations of the Path Validation mechanism described
in section 6 that omit revocation checking provide less assurance
than those that support it.
The path validation algorithm depends on the certain knowledge of the
public keys (and other information) about one or more trusted CAs.
The decision to trust a CA is an important decision as it ultimately
determines the trust afforded a certificate. The authenticated
distribution of trusted CA public keys (usually in the form of a
"self-signed" certificate) is a security critical out of band process
that is beyond the scope of this specification.
In addition, where a key compromise or CA failure occurs for a
trusted CA, the user will need to modify the information provided to
the path validation routine. Selection of too many trusted CAs will
make the trusted CA information difficult to maintain. On the other
hand, selection of only one trusted CA may limit users to a closed
community of users until a global PKI emerges.
The quality of implementations that process certificates may also
affect the degree of assurance provided. The path validation
algorithm described in section 6 relies upon the integrity of the
trusted CA information, and especially the integrity of the public
keys associated with the trusted CAs. By substituting public keys
for which an attacker has the private key, an attacker could trick
the user into accepting false certificates.
The binding between a key and certificate subject cannot be stronger
than the cryptographic module implementation and algorithms used to
generate the signature. Short key lengths or weak hash algorithms
will limit the utility of a certificate. CAs are encouraged to note
advances in cryptology so they can employ strong cryptographic
techniques. In addition, CAs should decline to issue certificates to
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RFC 2459 Internet X.509 Public Key Infrastructure January 1999
CAs or end entities that generate weak signatures.
Inconsistent application of name comparison rules may result in
acceptance of invalid X.509 certification paths, or rejection of
valid ones. The X.500 series of specifications defines rules for
comparing distinguished names require comparison of strings without
regard to case, character set, multi-character white space substring,
or leading and trailing white space. This specification relaxes
these requirements, requiring support for binary comparison at a
minimum.
CAs shall encode the distinguished name in the subject field of a CA
certificate identically to the distinguished name in the issuer field
in certificates issued by the latter CA. If CAs use different
encodings, implementations of this specification may fail to
recognize name chains for paths that include this certificate. As a
consequence, valid paths could be rejected.
In addition, name constraints for distinguished names shall be stated
identically to the encoding used in the subject field or
subjectAltName extension. If not, (1) name constraints stated as
excludedSubTrees will not match and invalid paths will be accepted
and (2) name constraints expressed as permittedSubtrees will not
match and valid paths will be rejected. To avoid acceptance of
invalid paths, CAs should state name constraints for distinguished
names as permittedSubtrees where ever possible.
Housley, et. al. Standards Track [Page 69]

RFC 2459 Internet X.509 Public Key Infrastructure January 1999Appendix A. Psuedo-ASN.1 Structures and OIDs
This section describes data objects used by conforming PKI components
in an "ASN.1-like" syntax. This syntax is a hybrid of the 1988 and
1993 ASN.1 syntaxes. The 1988 ASN.1 syntax is augmented with 1993
UNIVERSAL Types UniversalString, BMPString and UTF8String.
The ASN.1 syntax does not permit the inclusion of type statements in
the ASN.1 module, and the 1993 ASN.1 standard does not permit use of
the new UNIVERSAL types in modules using the 1988 syntax. As a
result, this module does not conform to either version of the ASN.1
standard.
This appendix may be converted into 1988 ASN.1 by replacing the
defintions for the UNIVERSAL Types with the 1988 catch-all "ANY".
A.1 Explicitly Tagged Module, 1988 Syntax
PKIX1Explicit88 {iso(1) identified-organization(3) dod(6) internet(1)
security(5) mechanisms(5) pkix(7) id-mod(0) id-pkix1-explicit-88(1)}
DEFINITIONS EXPLICIT TAGS ::=
BEGIN
-- EXPORTS ALL --
-- IMPORTS NONE --
-- UNIVERSAL Types defined in '93 and '98 ASN.1
-- but required by this specification
UniversalString ::= [UNIVERSAL 28] IMPLICIT OCTET STRING
-- UniversalString is defined in ASN.1:1993
BMPString ::= [UNIVERSAL 30] IMPLICIT OCTET STRING
-- BMPString is the subtype of UniversalString and models
-- the Basic Multilingual Plane of ISO/IEC/ITU 10646-1
UTF8String ::= [UNIVERSAL 12] IMPLICIT OCTET STRING
-- The content of this type conforms to RFC 2279.
--
-- PKIX specific OIDs
id-pkix OBJECT IDENTIFIER ::=
{ iso(1) identified-organization(3) dod(6) internet(1)
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RFC 2459 Internet X.509 Public Key Infrastructure January 1999
-- such a value. As a minimum, 16 octets, or twice the specified upper
-- bound, whichever is the larger, should be allowed for TeletexString.
-- For UTF8String or UniversalString at least four times the upper
-- bound should be allowed.
END
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RFC 2459 Internet X.509 Public Key Infrastructure January 1999
ub-x121-address-length INTEGER ::= 16
-- Note - upper bounds on TeletexString are measured in characters.
-- A significantly greater number of octets will be required to hold
-- such a value. As a minimum, 16 octets, or twice the specified upper
-- bound, whichever is the larger, should be allowed.
END
Housley, et. al. Standards Track [Page 107]

RFC 2459 Internet X.509 Public Key Infrastructure January 1999Appendix C. ASN.1 Notes
The construct "SEQUENCE SIZE (1..MAX) OF" appears in several ASN.1
constructs. A valid ASN.1 sequence will have zero or more entries.
The SIZE (1..MAX) construct constrains the sequence to have at least
one entry. MAX indicates the upper bound is unspecified.
Implementations are free to choose an upper bound that suits their
environment.
The construct "positiveInt ::= INTEGER (0..MAX)" defines positiveInt
as a subtype of INTEGER containing integers greater than or equal to
zero. The upper bound is unspecified. Implementations are free to
select an upper bound that suits their environment.
The character string type PrintableString supports a very basic Latin
character set: the lower case letters 'a' through 'z', upper case
letters 'A' through 'Z', the digits '0' through '9', eleven special
characters ' " ( ) + , - . / : ? and space.
The character string type TeletexString is a superset of
PrintableString. TeletexString supports a fairly standard (ascii-
like) Latin character set, Latin characters with non-spacing accents
and Japanese characters.
The character string type UniversalString supports any of the
characters allowed by ISO 10646-1. ISO 10646 is the Universal
multiple-octet coded Character Set (UCS). ISO 10646-1 specifes the
architecture and the "basic multilingual plane" - a large standard
character set which includes all major world character standards.
The character string type UTF8String will be introduced in the 1998
version of ASN.1. UTF8String is a universal type and has been
assigned tag number 12. The content of UTF8String was defined by RFC2044 and updated in RFC 2279, "UTF-8, a transformation Format of ISP
10646." ISO is expected to formally add UTF8String to the list of
choices for DirectoryString in 1998 as well.
In anticipation of these changes, and in conformance with IETF Best
Practices codified in RFC 2277, IETF Policy on Character Sets and
Languages, this document includes UTF8String as a choice in
DirectoryString and the CPS qualifier extensions.
Housley, et. al. Standards Track [Page 116]

RFC 2459 Internet X.509 Public Key Infrastructure January 1999Appendix D. Examples
This section contains four examples: three certificates and a CRL.
The first two certificates and the CRL comprise a minimal
certification path.
Section D.1 contains an annotated hex dump of a "self-signed"
certificate issued by a CA whose distinguished name is
cn=us,o=gov,ou=nist. The certificate contains a DSA public key with
parameters, and is signed by the corresponding DSA private key.
Section D.2 contains an annotated hex dump of an end-entity
certificate. The end entity certificate contains a DSA public key,
and is signed by the private key corresponding to the "self-signed"
certificate in section D.1.
Section D.3 contains a dump of an end entity certificate which
contains an RSA public key and is signed with RSA and MD5. This
certificate is not part of the minimal certification path.
Section D.4 contains an annotated hex dump of a CRL. The CRL is
issued by the CA whose distinguished name is cn=us,o=gov,ou=nist and
the list of revoked certificates includes the end entity certificate
presented in D.2.
D.1 Certificate
This section contains an annotated hex dump of a 699 byte version 3
certificate. The certificate contains the following information:
(a) the serial number is 17 (11 hex);
(b) the certificate is signed with DSA and the SHA-1 hash algorithm;
(c) the issuer's distinguished name is OU=nist; O=gov; C=US
(d) and the subject's distinguished name is OU=nist; O=gov; C=US
(e) the certificate was issued on June 30, 1997 and will expire on
December 31, 1997;
(f) the certificate contains a 1024 bit DSA public key with
parameters;
(g) the certificate contains a subject key identifier extension; and
(h) the certificate is a CA certificate (as indicated through the
basic constraints extension.)
0000 30 82 02 b7 695: SEQUENCE
0004 30 82 02 77 631: . SEQUENCE tbscertificate
0008 a0 03 3: . . [0]
0010 02 01 1: . . . INTEGER 2
: 02
0013 02 01 1: . . INTEGER 17
: 11
Housley, et. al. Standards Track [Page 117]

RFC 2459 Internet X.509 Public Key Infrastructure January 1999Appendix F. Full Copyright Statement
Copyright (C) The Internet Society (1999). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Housley, et. al. Standards Track [Page 129]